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kernel/arch/x86/entry/entry_64.S
tqcq e5480701d3 init repo.
2024-07-22 17:22:30 +08:00

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/* SPDX-License-Identifier: GPL-2.0 */
/*
* linux/arch/x86_64/entry.S
*
* Copyright (C) 1991, 1992 Linus Torvalds
* Copyright (C) 2000, 2001, 2002 Andi Kleen SuSE Labs
* Copyright (C) 2000 Pavel Machek <pavel@suse.cz>
*
* entry.S contains the system-call and fault low-level handling routines.
*
* Some of this is documented in Documentation/x86/entry_64.rst
*
* A note on terminology:
* - iret frame: Architecture defined interrupt frame from SS to RIP
* at the top of the kernel process stack.
*
* Some macro usage:
* - SYM_FUNC_START/END:Define functions in the symbol table.
* - idtentry: Define exception entry points.
*/
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/cache.h>
#include <asm/errno.h>
#include <asm/asm-offsets.h>
#include <asm/msr.h>
#include <asm/unistd.h>
#include <asm/thread_info.h>
#include <asm/hw_irq.h>
#include <asm/page_types.h>
#include <asm/irqflags.h>
#include <asm/paravirt.h>
#include <asm/percpu.h>
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/pgtable_types.h>
#include <asm/export.h>
#include <asm/frame.h>
#include <asm/trapnr.h>
#include <asm/nospec-branch.h>
#include <asm/fsgsbase.h>
#include <linux/err.h>
#include "calling.h"
.code64
.section .entry.text, "ax"
/*
* 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
*
* This is the only entry point used for 64-bit system calls. The
* hardware interface is reasonably well designed and the register to
* argument mapping Linux uses fits well with the registers that are
* available when SYSCALL is used.
*
* SYSCALL instructions can be found inlined in libc implementations as
* well as some other programs and libraries. There are also a handful
* of SYSCALL instructions in the vDSO used, for example, as a
* clock_gettimeofday fallback.
*
* 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
* then loads new ss, cs, and rip from previously programmed MSRs.
* rflags gets masked by a value from another MSR (so CLD and CLAC
* are not needed). SYSCALL does not save anything on the stack
* and does not change rsp.
*
* Registers on entry:
* rax system call number
* rcx return address
* r11 saved rflags (note: r11 is callee-clobbered register in C ABI)
* rdi arg0
* rsi arg1
* rdx arg2
* r10 arg3 (needs to be moved to rcx to conform to C ABI)
* r8 arg4
* r9 arg5
* (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
*
* Only called from user space.
*
* When user can change pt_regs->foo always force IRET. That is because
* it deals with uncanonical addresses better. SYSRET has trouble
* with them due to bugs in both AMD and Intel CPUs.
*/
SYM_CODE_START(entry_SYSCALL_64)
UNWIND_HINT_ENTRY
swapgs
/* tss.sp2 is scratch space. */
movq %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL)
/* Construct struct pt_regs on stack */
pushq $__USER_DS /* pt_regs->ss */
pushq PER_CPU_VAR(cpu_tss_rw + TSS_sp2) /* pt_regs->sp */
pushq %r11 /* pt_regs->flags */
pushq $__USER_CS /* pt_regs->cs */
pushq %rcx /* pt_regs->ip */
SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
pushq %rax /* pt_regs->orig_ax */
PUSH_AND_CLEAR_REGS rax=$-ENOSYS
/* IRQs are off. */
movq %rsp, %rdi
/* Sign extend the lower 32bit as syscall numbers are treated as int */
movslq %eax, %rsi
/* clobbers %rax, make sure it is after saving the syscall nr */
IBRS_ENTER
UNTRAIN_RET
CLEAR_BRANCH_HISTORY
call do_syscall_64 /* returns with IRQs disabled */
/*
* Try to use SYSRET instead of IRET if we're returning to
* a completely clean 64-bit userspace context. If we're not,
* go to the slow exit path.
* In the Xen PV case we must use iret anyway.
*/
ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", \
X86_FEATURE_XENPV
movq RCX(%rsp), %rcx
movq RIP(%rsp), %r11
cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
jne swapgs_restore_regs_and_return_to_usermode
/*
* On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
* in kernel space. This essentially lets the user take over
* the kernel, since userspace controls RSP.
*
* If width of "canonical tail" ever becomes variable, this will need
* to be updated to remain correct on both old and new CPUs.
*
* Change top bits to match most significant bit (47th or 56th bit
* depending on paging mode) in the address.
*/
#ifdef CONFIG_X86_5LEVEL
ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
"shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
#else
shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
#endif
/* If this changed %rcx, it was not canonical */
cmpq %rcx, %r11
jne swapgs_restore_regs_and_return_to_usermode
cmpq $__USER_CS, CS(%rsp) /* CS must match SYSRET */
jne swapgs_restore_regs_and_return_to_usermode
movq R11(%rsp), %r11
cmpq %r11, EFLAGS(%rsp) /* R11 == RFLAGS */
jne swapgs_restore_regs_and_return_to_usermode
/*
* SYSRET cannot restore RF. It can restore TF, but unlike IRET,
* restoring TF results in a trap from userspace immediately after
* SYSRET.
*/
testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
jnz swapgs_restore_regs_and_return_to_usermode
/* nothing to check for RSP */
cmpq $__USER_DS, SS(%rsp) /* SS must match SYSRET */
jne swapgs_restore_regs_and_return_to_usermode
/*
* We win! This label is here just for ease of understanding
* perf profiles. Nothing jumps here.
*/
syscall_return_via_sysret:
IBRS_EXIT
POP_REGS pop_rdi=0
/*
* Now all regs are restored except RSP and RDI.
* Save old stack pointer and switch to trampoline stack.
*/
movq %rsp, %rdi
movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
UNWIND_HINT_EMPTY
pushq RSP-RDI(%rdi) /* RSP */
pushq (%rdi) /* RDI */
/*
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
STACKLEAK_ERASE_NOCLOBBER
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
popq %rdi
popq %rsp
swapgs
CLEAR_CPU_BUFFERS
sysretq
SYM_CODE_END(entry_SYSCALL_64)
/*
* %rdi: prev task
* %rsi: next task
*/
.pushsection .text, "ax"
SYM_FUNC_START(__switch_to_asm)
/*
* Save callee-saved registers
* This must match the order in inactive_task_frame
*/
pushq %rbp
pushq %rbx
pushq %r12
pushq %r13
pushq %r14
pushq %r15
/* switch stack */
movq %rsp, TASK_threadsp(%rdi)
movq TASK_threadsp(%rsi), %rsp
#ifdef CONFIG_STACKPROTECTOR
movq TASK_stack_canary(%rsi), %rbx
movq %rbx, PER_CPU_VAR(fixed_percpu_data) + stack_canary_offset
#endif
/*
* When switching from a shallower to a deeper call stack
* the RSB may either underflow or use entries populated
* with userspace addresses. On CPUs where those concerns
* exist, overwrite the RSB with entries which capture
* speculative execution to prevent attack.
*/
FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
/* restore callee-saved registers */
popq %r15
popq %r14
popq %r13
popq %r12
popq %rbx
popq %rbp
jmp __switch_to
SYM_FUNC_END(__switch_to_asm)
.popsection
/*
* A newly forked process directly context switches into this address.
*
* rax: prev task we switched from
* rbx: kernel thread func (NULL for user thread)
* r12: kernel thread arg
*/
.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork)
UNWIND_HINT_EMPTY
movq %rax, %rdi
call schedule_tail /* rdi: 'prev' task parameter */
testq %rbx, %rbx /* from kernel_thread? */
jnz 1f /* kernel threads are uncommon */
2:
UNWIND_HINT_REGS
movq %rsp, %rdi
call syscall_exit_to_user_mode /* returns with IRQs disabled */
jmp swapgs_restore_regs_and_return_to_usermode
1:
/* kernel thread */
UNWIND_HINT_EMPTY
movq %r12, %rdi
CALL_NOSPEC rbx
/*
* A kernel thread is allowed to return here after successfully
* calling kernel_execve(). Exit to userspace to complete the execve()
* syscall.
*/
movq $0, RAX(%rsp)
jmp 2b
SYM_CODE_END(ret_from_fork)
.popsection
.macro DEBUG_ENTRY_ASSERT_IRQS_OFF
#ifdef CONFIG_DEBUG_ENTRY
pushq %rax
SAVE_FLAGS
testl $X86_EFLAGS_IF, %eax
jz .Lokay_\@
ud2
.Lokay_\@:
popq %rax
#endif
.endm
SYM_CODE_START_LOCAL(xen_error_entry)
UNWIND_HINT_FUNC
PUSH_AND_CLEAR_REGS save_ret=1
ENCODE_FRAME_POINTER 8
UNTRAIN_RET
RET
SYM_CODE_END(xen_error_entry)
/**
* idtentry_body - Macro to emit code calling the C function
* @cfunc: C function to be called
* @has_error_code: Hardware pushed error code on stack
*/
.macro idtentry_body cfunc has_error_code:req
/*
* Call error_entry() and switch to the task stack if from userspace.
*
* When in XENPV, it is already in the task stack, and it can't fault
* for native_iret() nor native_load_gs_index() since XENPV uses its
* own pvops for IRET and load_gs_index(). And it doesn't need to
* switch the CR3. So it can skip invoking error_entry().
*/
ALTERNATIVE "call error_entry; movq %rax, %rsp", \
"call xen_error_entry", X86_FEATURE_XENPV
ENCODE_FRAME_POINTER
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs pointer into 1st argument*/
.if \has_error_code == 1
movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
.endif
call \cfunc
jmp error_return
.endm
/**
* idtentry - Macro to generate entry stubs for simple IDT entries
* @vector: Vector number
* @asmsym: ASM symbol for the entry point
* @cfunc: C function to be called
* @has_error_code: Hardware pushed error code on stack
*
* The macro emits code to set up the kernel context for straight forward
* and simple IDT entries. No IST stack, no paranoid entry checks.
*/
.macro idtentry vector asmsym cfunc has_error_code:req
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS offset=\has_error_code*8
ASM_CLAC
cld
.if \has_error_code == 0
pushq $-1 /* ORIG_RAX: no syscall to restart */
.endif
.if \vector == X86_TRAP_BP
/*
* If coming from kernel space, create a 6-word gap to allow the
* int3 handler to emulate a call instruction.
*/
testb $3, CS-ORIG_RAX(%rsp)
jnz .Lfrom_usermode_no_gap_\@
.rept 6
pushq 5*8(%rsp)
.endr
UNWIND_HINT_IRET_REGS offset=8
.Lfrom_usermode_no_gap_\@:
.endif
idtentry_body \cfunc \has_error_code
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
/*
* Interrupt entry/exit.
*
+ The interrupt stubs push (vector) onto the stack, which is the error_code
* position of idtentry exceptions, and jump to one of the two idtentry points
* (common/spurious).
*
* common_interrupt is a hotpath, align it to a cache line
*/
.macro idtentry_irq vector cfunc
.p2align CONFIG_X86_L1_CACHE_SHIFT
idtentry \vector asm_\cfunc \cfunc has_error_code=1
.endm
/*
* System vectors which invoke their handlers directly and are not
* going through the regular common device interrupt handling code.
*/
.macro idtentry_sysvec vector cfunc
idtentry \vector asm_\cfunc \cfunc has_error_code=0
.endm
/**
* idtentry_mce_db - Macro to generate entry stubs for #MC and #DB
* @vector: Vector number
* @asmsym: ASM symbol for the entry point
* @cfunc: C function to be called
*
* The macro emits code to set up the kernel context for #MC and #DB
*
* If the entry comes from user space it uses the normal entry path
* including the return to user space work and preemption checks on
* exit.
*
* If hits in kernel mode then it needs to go through the paranoid
* entry as the exception can hit any random state. No preemption
* check on exit to keep the paranoid path simple.
*/
.macro idtentry_mce_db vector asmsym cfunc
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS
ASM_CLAC
cld
pushq $-1 /* ORIG_RAX: no syscall to restart */
/*
* If the entry is from userspace, switch stacks and treat it as
* a normal entry.
*/
testb $3, CS-ORIG_RAX(%rsp)
jnz .Lfrom_usermode_switch_stack_\@
/* paranoid_entry returns GS information for paranoid_exit in EBX. */
call paranoid_entry
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs pointer */
call \cfunc
jmp paranoid_exit
/* Switch to the regular task stack and use the noist entry point */
.Lfrom_usermode_switch_stack_\@:
idtentry_body noist_\cfunc, has_error_code=0
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
#ifdef CONFIG_AMD_MEM_ENCRYPT
/**
* idtentry_vc - Macro to generate entry stub for #VC
* @vector: Vector number
* @asmsym: ASM symbol for the entry point
* @cfunc: C function to be called
*
* The macro emits code to set up the kernel context for #VC. The #VC handler
* runs on an IST stack and needs to be able to cause nested #VC exceptions.
*
* To make this work the #VC entry code tries its best to pretend it doesn't use
* an IST stack by switching to the task stack if coming from user-space (which
* includes early SYSCALL entry path) or back to the stack in the IRET frame if
* entered from kernel-mode.
*
* If entered from kernel-mode the return stack is validated first, and if it is
* not safe to use (e.g. because it points to the entry stack) the #VC handler
* will switch to a fall-back stack (VC2) and call a special handler function.
*
* The macro is only used for one vector, but it is planned to be extended in
* the future for the #HV exception.
*/
.macro idtentry_vc vector asmsym cfunc
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS
ASM_CLAC
cld
/*
* If the entry is from userspace, switch stacks and treat it as
* a normal entry.
*/
testb $3, CS-ORIG_RAX(%rsp)
jnz .Lfrom_usermode_switch_stack_\@
/*
* paranoid_entry returns SWAPGS flag for paranoid_exit in EBX.
* EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS
*/
call paranoid_entry
UNWIND_HINT_REGS
/*
* Switch off the IST stack to make it free for nested exceptions. The
* vc_switch_off_ist() function will switch back to the interrupted
* stack if it is safe to do so. If not it switches to the VC fall-back
* stack.
*/
movq %rsp, %rdi /* pt_regs pointer */
call vc_switch_off_ist
movq %rax, %rsp /* Switch to new stack */
ENCODE_FRAME_POINTER
UNWIND_HINT_REGS
/* Update pt_regs */
movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
movq %rsp, %rdi /* pt_regs pointer */
call kernel_\cfunc
/*
* No need to switch back to the IST stack. The current stack is either
* identical to the stack in the IRET frame or the VC fall-back stack,
* so it is definitely mapped even with PTI enabled.
*/
jmp paranoid_exit
/* Switch to the regular task stack */
.Lfrom_usermode_switch_stack_\@:
idtentry_body user_\cfunc, has_error_code=1
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
#endif
/*
* Double fault entry. Straight paranoid. No checks from which context
* this comes because for the espfix induced #DF this would do the wrong
* thing.
*/
.macro idtentry_df vector asmsym cfunc
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS offset=8
ASM_CLAC
cld
/* paranoid_entry returns GS information for paranoid_exit in EBX. */
call paranoid_entry
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs pointer into first argument */
movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
call \cfunc
jmp paranoid_exit
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
/*
* Include the defines which emit the idt entries which are shared
* shared between 32 and 64 bit and emit the __irqentry_text_* markers
* so the stacktrace boundary checks work.
*/
.align 16
.globl __irqentry_text_start
__irqentry_text_start:
#include <asm/idtentry.h>
.align 16
.globl __irqentry_text_end
__irqentry_text_end:
SYM_CODE_START_LOCAL(common_interrupt_return)
SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
IBRS_EXIT
#ifdef CONFIG_DEBUG_ENTRY
/* Assert that pt_regs indicates user mode. */
testb $3, CS(%rsp)
jnz 1f
ud2
1:
#endif
#ifdef CONFIG_XEN_PV
ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
#endif
POP_REGS pop_rdi=0
/*
* The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
* Save old stack pointer and switch to trampoline stack.
*/
movq %rsp, %rdi
movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
UNWIND_HINT_EMPTY
/* Copy the IRET frame to the trampoline stack. */
pushq 6*8(%rdi) /* SS */
pushq 5*8(%rdi) /* RSP */
pushq 4*8(%rdi) /* EFLAGS */
pushq 3*8(%rdi) /* CS */
pushq 2*8(%rdi) /* RIP */
/* Push user RDI on the trampoline stack. */
pushq (%rdi)
/*
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
STACKLEAK_ERASE_NOCLOBBER
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
/* Restore RDI. */
popq %rdi
SWAPGS
CLEAR_CPU_BUFFERS
INTERRUPT_RETURN
SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)
#ifdef CONFIG_DEBUG_ENTRY
/* Assert that pt_regs indicates kernel mode. */
testb $3, CS(%rsp)
jz 1f
ud2
1:
#endif
POP_REGS
addq $8, %rsp /* skip regs->orig_ax */
/*
* ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
* when returning from IPI handler.
*/
INTERRUPT_RETURN
SYM_INNER_LABEL_ALIGN(native_iret, SYM_L_GLOBAL)
UNWIND_HINT_IRET_REGS
/*
* Are we returning to a stack segment from the LDT? Note: in
* 64-bit mode SS:RSP on the exception stack is always valid.
*/
#ifdef CONFIG_X86_ESPFIX64
testb $4, (SS-RIP)(%rsp)
jnz native_irq_return_ldt
#endif
SYM_INNER_LABEL(native_irq_return_iret, SYM_L_GLOBAL)
/*
* This may fault. Non-paranoid faults on return to userspace are
* handled by fixup_bad_iret. These include #SS, #GP, and #NP.
* Double-faults due to espfix64 are handled in exc_double_fault.
* Other faults here are fatal.
*/
iretq
#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
/*
* We are running with user GSBASE. All GPRs contain their user
* values. We have a percpu ESPFIX stack that is eight slots
* long (see ESPFIX_STACK_SIZE). espfix_waddr points to the bottom
* of the ESPFIX stack.
*
* We clobber RAX and RDI in this code. We stash RDI on the
* normal stack and RAX on the ESPFIX stack.
*
* The ESPFIX stack layout we set up looks like this:
*
* --- top of ESPFIX stack ---
* SS
* RSP
* RFLAGS
* CS
* RIP <-- RSP points here when we're done
* RAX <-- espfix_waddr points here
* --- bottom of ESPFIX stack ---
*/
pushq %rdi /* Stash user RDI */
swapgs /* to kernel GS */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */
UNTRAIN_RET
movq PER_CPU_VAR(espfix_waddr), %rdi
movq %rax, (0*8)(%rdi) /* user RAX */
movq (1*8)(%rsp), %rax /* user RIP */
movq %rax, (1*8)(%rdi)
movq (2*8)(%rsp), %rax /* user CS */
movq %rax, (2*8)(%rdi)
movq (3*8)(%rsp), %rax /* user RFLAGS */
movq %rax, (3*8)(%rdi)
movq (5*8)(%rsp), %rax /* user SS */
movq %rax, (5*8)(%rdi)
movq (4*8)(%rsp), %rax /* user RSP */
movq %rax, (4*8)(%rdi)
/* Now RAX == RSP. */
andl $0xffff0000, %eax /* RAX = (RSP & 0xffff0000) */
/*
* espfix_stack[31:16] == 0. The page tables are set up such that
* (espfix_stack | (X & 0xffff0000)) points to a read-only alias of
* espfix_waddr for any X. That is, there are 65536 RO aliases of
* the same page. Set up RSP so that RSP[31:16] contains the
* respective 16 bits of the /userspace/ RSP and RSP nonetheless
* still points to an RO alias of the ESPFIX stack.
*/
orq PER_CPU_VAR(espfix_stack), %rax
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
swapgs /* to user GS */
popq %rdi /* Restore user RDI */
movq %rax, %rsp
UNWIND_HINT_IRET_REGS offset=8
/*
* At this point, we cannot write to the stack any more, but we can
* still read.
*/
popq %rax /* Restore user RAX */
CLEAR_CPU_BUFFERS
/*
* RSP now points to an ordinary IRET frame, except that the page
* is read-only and RSP[31:16] are preloaded with the userspace
* values. We can now IRET back to userspace.
*/
jmp native_irq_return_iret
#endif
SYM_CODE_END(common_interrupt_return)
_ASM_NOKPROBE(common_interrupt_return)
/*
* Reload gs selector with exception handling
* edi: new selector
*
* Is in entry.text as it shouldn't be instrumented.
*/
SYM_FUNC_START(asm_load_gs_index)
FRAME_BEGIN
swapgs
.Lgs_change:
movl %edi, %gs
2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
swapgs
FRAME_END
RET
SYM_FUNC_END(asm_load_gs_index)
EXPORT_SYMBOL(asm_load_gs_index)
_ASM_EXTABLE(.Lgs_change, .Lbad_gs)
.section .fixup, "ax"
/* running with kernelgs */
SYM_CODE_START_LOCAL_NOALIGN(.Lbad_gs)
swapgs /* switch back to user gs */
.macro ZAP_GS
/* This can't be a string because the preprocessor needs to see it. */
movl $__USER_DS, %eax
movl %eax, %gs
.endm
ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
xorl %eax, %eax
movl %eax, %gs
jmp 2b
SYM_CODE_END(.Lbad_gs)
.previous
#ifdef CONFIG_XEN_PV
/*
* A note on the "critical region" in our callback handler.
* We want to avoid stacking callback handlers due to events occurring
* during handling of the last event. To do this, we keep events disabled
* until we've done all processing. HOWEVER, we must enable events before
* popping the stack frame (can't be done atomically) and so it would still
* be possible to get enough handler activations to overflow the stack.
* Although unlikely, bugs of that kind are hard to track down, so we'd
* like to avoid the possibility.
* So, on entry to the handler we detect whether we interrupted an
* existing activation in its critical region -- if so, we pop the current
* activation and restart the handler using the previous one.
*
* C calling convention: exc_xen_hypervisor_callback(struct *pt_regs)
*/
SYM_CODE_START_LOCAL(exc_xen_hypervisor_callback)
/*
* Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
* see the correct pointer to the pt_regs
*/
UNWIND_HINT_FUNC
movq %rdi, %rsp /* we don't return, adjust the stack frame */
UNWIND_HINT_REGS
call xen_pv_evtchn_do_upcall
jmp error_return
SYM_CODE_END(exc_xen_hypervisor_callback)
/*
* Hypervisor uses this for application faults while it executes.
* We get here for two reasons:
* 1. Fault while reloading DS, ES, FS or GS
* 2. Fault while executing IRET
* Category 1 we do not need to fix up as Xen has already reloaded all segment
* registers that could be reloaded and zeroed the others.
* Category 2 we fix up by killing the current process. We cannot use the
* normal Linux return path in this case because if we use the IRET hypercall
* to pop the stack frame we end up in an infinite loop of failsafe callbacks.
* We distinguish between categories by comparing each saved segment register
* with its current contents: any discrepancy means we in category 1.
*/
SYM_CODE_START(xen_failsafe_callback)
UNWIND_HINT_EMPTY
movl %ds, %ecx
cmpw %cx, 0x10(%rsp)
jne 1f
movl %es, %ecx
cmpw %cx, 0x18(%rsp)
jne 1f
movl %fs, %ecx
cmpw %cx, 0x20(%rsp)
jne 1f
movl %gs, %ecx
cmpw %cx, 0x28(%rsp)
jne 1f
/* All segments match their saved values => Category 2 (Bad IRET). */
movq (%rsp), %rcx
movq 8(%rsp), %r11
addq $0x30, %rsp
pushq $0 /* RIP */
UNWIND_HINT_IRET_REGS offset=8
jmp asm_exc_general_protection
1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
movq (%rsp), %rcx
movq 8(%rsp), %r11
addq $0x30, %rsp
UNWIND_HINT_IRET_REGS
pushq $-1 /* orig_ax = -1 => not a system call */
PUSH_AND_CLEAR_REGS
ENCODE_FRAME_POINTER
jmp error_return
SYM_CODE_END(xen_failsafe_callback)
#endif /* CONFIG_XEN_PV */
/*
* Save all registers in pt_regs. Return GSBASE related information
* in EBX depending on the availability of the FSGSBASE instructions:
*
* FSGSBASE R/EBX
* N 0 -> SWAPGS on exit
* 1 -> no SWAPGS on exit
*
* Y GSBASE value at entry, must be restored in paranoid_exit
*
* R14 - old CR3
* R15 - old SPEC_CTRL
*/
SYM_CODE_START_LOCAL(paranoid_entry)
UNWIND_HINT_FUNC
PUSH_AND_CLEAR_REGS save_ret=1
ENCODE_FRAME_POINTER 8
/*
* Always stash CR3 in %r14. This value will be restored,
* verbatim, at exit. Needed if paranoid_entry interrupted
* another entry that already switched to the user CR3 value
* but has not yet returned to userspace.
*
* This is also why CS (stashed in the "iret frame" by the
* hardware at entry) can not be used: this may be a return
* to kernel code, but with a user CR3 value.
*
* Switching CR3 does not depend on kernel GSBASE so it can
* be done before switching to the kernel GSBASE. This is
* required for FSGSBASE because the kernel GSBASE has to
* be retrieved from a kernel internal table.
*/
SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
/*
* Handling GSBASE depends on the availability of FSGSBASE.
*
* Without FSGSBASE the kernel enforces that negative GSBASE
* values indicate kernel GSBASE. With FSGSBASE no assumptions
* can be made about the GSBASE value when entering from user
* space.
*/
ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE
/*
* Read the current GSBASE and store it in %rbx unconditionally,
* retrieve and set the current CPUs kernel GSBASE. The stored value
* has to be restored in paranoid_exit unconditionally.
*
* The unconditional write to GS base below ensures that no subsequent
* loads based on a mispredicted GS base can happen, therefore no LFENCE
* is needed here.
*/
SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
jmp .Lparanoid_gsbase_done
.Lparanoid_entry_checkgs:
/* EBX = 1 -> kernel GSBASE active, no restore required */
movl $1, %ebx
/*
* The kernel-enforced convention is a negative GSBASE indicates
* a kernel value. No SWAPGS needed on entry and exit.
*/
movl $MSR_GS_BASE, %ecx
rdmsr
testl %edx, %edx
js .Lparanoid_kernel_gsbase
/* EBX = 0 -> SWAPGS required on exit */
xorl %ebx, %ebx
swapgs
.Lparanoid_kernel_gsbase:
FENCE_SWAPGS_KERNEL_ENTRY
.Lparanoid_gsbase_done:
/*
* Once we have CR3 and %GS setup save and set SPEC_CTRL. Just like
* CR3 above, keep the old value in a callee saved register.
*/
IBRS_ENTER save_reg=%r15
UNTRAIN_RET
RET
SYM_CODE_END(paranoid_entry)
/*
* "Paranoid" exit path from exception stack. This is invoked
* only on return from non-NMI IST interrupts that came
* from kernel space.
*
* We may be returning to very strange contexts (e.g. very early
* in syscall entry), so checking for preemption here would
* be complicated. Fortunately, there's no good reason to try
* to handle preemption here.
*
* R/EBX contains the GSBASE related information depending on the
* availability of the FSGSBASE instructions:
*
* FSGSBASE R/EBX
* N 0 -> SWAPGS on exit
* 1 -> no SWAPGS on exit
*
* Y User space GSBASE, must be restored unconditionally
*
* R14 - old CR3
* R15 - old SPEC_CTRL
*/
SYM_CODE_START_LOCAL(paranoid_exit)
UNWIND_HINT_REGS
/*
* Must restore IBRS state before both CR3 and %GS since we need access
* to the per-CPU x86_spec_ctrl_shadow variable.
*/
IBRS_EXIT save_reg=%r15
/*
* The order of operations is important. RESTORE_CR3 requires
* kernel GSBASE.
*
* NB to anyone to try to optimize this code: this code does
* not execute at all for exceptions from user mode. Those
* exceptions go through error_exit instead.
*/
RESTORE_CR3 scratch_reg=%rax save_reg=%r14
/* Handle the three GSBASE cases */
ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE
/* With FSGSBASE enabled, unconditionally restore GSBASE */
wrgsbase %rbx
jmp restore_regs_and_return_to_kernel
.Lparanoid_exit_checkgs:
/* On non-FSGSBASE systems, conditionally do SWAPGS */
testl %ebx, %ebx
jnz restore_regs_and_return_to_kernel
/* We are returning to a context with user GSBASE */
swapgs
jmp restore_regs_and_return_to_kernel
SYM_CODE_END(paranoid_exit)
/*
* Switch GS and CR3 if needed.
*/
SYM_CODE_START_LOCAL(error_entry)
UNWIND_HINT_FUNC
PUSH_AND_CLEAR_REGS save_ret=1
ENCODE_FRAME_POINTER 8
testb $3, CS+8(%rsp)
jz .Lerror_kernelspace
/*
* We entered from user mode or we're pretending to have entered
* from user mode due to an IRET fault.
*/
SWAPGS
FENCE_SWAPGS_USER_ENTRY
/* We have user CR3. Change to kernel CR3. */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
IBRS_ENTER
UNTRAIN_RET
leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
.Lerror_entry_from_usermode_after_swapgs:
/* Put us onto the real thread stack. */
call sync_regs
RET
/*
* There are two places in the kernel that can potentially fault with
* usergs. Handle them here. B stepping K8s sometimes report a
* truncated RIP for IRET exceptions returning to compat mode. Check
* for these here too.
*/
.Lerror_kernelspace:
leaq native_irq_return_iret(%rip), %rcx
cmpq %rcx, RIP+8(%rsp)
je .Lerror_bad_iret
movl %ecx, %eax /* zero extend */
cmpq %rax, RIP+8(%rsp)
je .Lbstep_iret
cmpq $.Lgs_change, RIP+8(%rsp)
jne .Lerror_entry_done_lfence
/*
* hack: .Lgs_change can fail with user gsbase. If this happens, fix up
* gsbase and proceed. We'll fix up the exception and land in
* .Lgs_change's error handler with kernel gsbase.
*/
SWAPGS
/*
* Issue an LFENCE to prevent GS speculation, regardless of whether it is a
* kernel or user gsbase.
*/
.Lerror_entry_done_lfence:
FENCE_SWAPGS_KERNEL_ENTRY
leaq 8(%rsp), %rax /* return pt_regs pointer */
ANNOTATE_UNRET_END
RET
.Lbstep_iret:
/* Fix truncated RIP */
movq %rcx, RIP+8(%rsp)
/* fall through */
.Lerror_bad_iret:
/*
* We came from an IRET to user mode, so we have user
* gsbase and CR3. Switch to kernel gsbase and CR3:
*/
SWAPGS
FENCE_SWAPGS_USER_ENTRY
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
IBRS_ENTER
UNTRAIN_RET
/*
* Pretend that the exception came from user mode: set up pt_regs
* as if we faulted immediately after IRET.
*/
leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
call fixup_bad_iret
mov %rax, %rdi
jmp .Lerror_entry_from_usermode_after_swapgs
SYM_CODE_END(error_entry)
SYM_CODE_START_LOCAL(error_return)
UNWIND_HINT_REGS
DEBUG_ENTRY_ASSERT_IRQS_OFF
testb $3, CS(%rsp)
jz restore_regs_and_return_to_kernel
jmp swapgs_restore_regs_and_return_to_usermode
SYM_CODE_END(error_return)
/*
* Runs on exception stack. Xen PV does not go through this path at all,
* so we can use real assembly here.
*
* Registers:
* %r14: Used to save/restore the CR3 of the interrupted context
* when PAGE_TABLE_ISOLATION is in use. Do not clobber.
*/
SYM_CODE_START(asm_exc_nmi)
UNWIND_HINT_IRET_REGS
/*
* We allow breakpoints in NMIs. If a breakpoint occurs, then
* the iretq it performs will take us out of NMI context.
* This means that we can have nested NMIs where the next
* NMI is using the top of the stack of the previous NMI. We
* can't let it execute because the nested NMI will corrupt the
* stack of the previous NMI. NMI handlers are not re-entrant
* anyway.
*
* To handle this case we do the following:
* Check the a special location on the stack that contains
* a variable that is set when NMIs are executing.
* The interrupted task's stack is also checked to see if it
* is an NMI stack.
* If the variable is not set and the stack is not the NMI
* stack then:
* o Set the special variable on the stack
* o Copy the interrupt frame into an "outermost" location on the
* stack
* o Copy the interrupt frame into an "iret" location on the stack
* o Continue processing the NMI
* If the variable is set or the previous stack is the NMI stack:
* o Modify the "iret" location to jump to the repeat_nmi
* o return back to the first NMI
*
* Now on exit of the first NMI, we first clear the stack variable
* The NMI stack will tell any nested NMIs at that point that it is
* nested. Then we pop the stack normally with iret, and if there was
* a nested NMI that updated the copy interrupt stack frame, a
* jump will be made to the repeat_nmi code that will handle the second
* NMI.
*
* However, espfix prevents us from directly returning to userspace
* with a single IRET instruction. Similarly, IRET to user mode
* can fault. We therefore handle NMIs from user space like
* other IST entries.
*/
ASM_CLAC
cld
/* Use %rdx as our temp variable throughout */
pushq %rdx
testb $3, CS-RIP+8(%rsp)
jz .Lnmi_from_kernel
/*
* NMI from user mode. We need to run on the thread stack, but we
* can't go through the normal entry paths: NMIs are masked, and
* we don't want to enable interrupts, because then we'll end
* up in an awkward situation in which IRQs are on but NMIs
* are off.
*
* We also must not push anything to the stack before switching
* stacks lest we corrupt the "NMI executing" variable.
*/
swapgs
FENCE_SWAPGS_USER_ENTRY
SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
movq %rsp, %rdx
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
UNWIND_HINT_IRET_REGS base=%rdx offset=8
pushq 5*8(%rdx) /* pt_regs->ss */
pushq 4*8(%rdx) /* pt_regs->rsp */
pushq 3*8(%rdx) /* pt_regs->flags */
pushq 2*8(%rdx) /* pt_regs->cs */
pushq 1*8(%rdx) /* pt_regs->rip */
UNWIND_HINT_IRET_REGS
pushq $-1 /* pt_regs->orig_ax */
PUSH_AND_CLEAR_REGS rdx=(%rdx)
ENCODE_FRAME_POINTER
IBRS_ENTER
UNTRAIN_RET
/*
* At this point we no longer need to worry about stack damage
* due to nesting -- we're on the normal thread stack and we're
* done with the NMI stack.
*/
movq %rsp, %rdi
movq $-1, %rsi
call exc_nmi
/*
* Return back to user mode. We must *not* do the normal exit
* work, because we don't want to enable interrupts.
*/
jmp swapgs_restore_regs_and_return_to_usermode
.Lnmi_from_kernel:
/*
* Here's what our stack frame will look like:
* +---------------------------------------------------------+
* | original SS |
* | original Return RSP |
* | original RFLAGS |
* | original CS |
* | original RIP |
* +---------------------------------------------------------+
* | temp storage for rdx |
* +---------------------------------------------------------+
* | "NMI executing" variable |
* +---------------------------------------------------------+
* | iret SS } Copied from "outermost" frame |
* | iret Return RSP } on each loop iteration; overwritten |
* | iret RFLAGS } by a nested NMI to force another |
* | iret CS } iteration if needed. |
* | iret RIP } |
* +---------------------------------------------------------+
* | outermost SS } initialized in first_nmi; |
* | outermost Return RSP } will not be changed before |
* | outermost RFLAGS } NMI processing is done. |
* | outermost CS } Copied to "iret" frame on each |
* | outermost RIP } iteration. |
* +---------------------------------------------------------+
* | pt_regs |
* +---------------------------------------------------------+
*
* The "original" frame is used by hardware. Before re-enabling
* NMIs, we need to be done with it, and we need to leave enough
* space for the asm code here.
*
* We return by executing IRET while RSP points to the "iret" frame.
* That will either return for real or it will loop back into NMI
* processing.
*
* The "outermost" frame is copied to the "iret" frame on each
* iteration of the loop, so each iteration starts with the "iret"
* frame pointing to the final return target.
*/
/*
* Determine whether we're a nested NMI.
*
* If we interrupted kernel code between repeat_nmi and
* end_repeat_nmi, then we are a nested NMI. We must not
* modify the "iret" frame because it's being written by
* the outer NMI. That's okay; the outer NMI handler is
* about to about to call exc_nmi() anyway, so we can just
* resume the outer NMI.
*/
movq $repeat_nmi, %rdx
cmpq 8(%rsp), %rdx
ja 1f
movq $end_repeat_nmi, %rdx
cmpq 8(%rsp), %rdx
ja nested_nmi_out
1:
/*
* Now check "NMI executing". If it's set, then we're nested.
* This will not detect if we interrupted an outer NMI just
* before IRET.
*/
cmpl $1, -8(%rsp)
je nested_nmi
/*
* Now test if the previous stack was an NMI stack. This covers
* the case where we interrupt an outer NMI after it clears
* "NMI executing" but before IRET. We need to be careful, though:
* there is one case in which RSP could point to the NMI stack
* despite there being no NMI active: naughty userspace controls
* RSP at the very beginning of the SYSCALL targets. We can
* pull a fast one on naughty userspace, though: we program
* SYSCALL to mask DF, so userspace cannot cause DF to be set
* if it controls the kernel's RSP. We set DF before we clear
* "NMI executing".
*/
lea 6*8(%rsp), %rdx
/* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
cmpq %rdx, 4*8(%rsp)
/* If the stack pointer is above the NMI stack, this is a normal NMI */
ja first_nmi
subq $EXCEPTION_STKSZ, %rdx
cmpq %rdx, 4*8(%rsp)
/* If it is below the NMI stack, it is a normal NMI */
jb first_nmi
/* Ah, it is within the NMI stack. */
testb $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp)
jz first_nmi /* RSP was user controlled. */
/* This is a nested NMI. */
nested_nmi:
/*
* Modify the "iret" frame to point to repeat_nmi, forcing another
* iteration of NMI handling.
*/
subq $8, %rsp
leaq -10*8(%rsp), %rdx
pushq $__KERNEL_DS
pushq %rdx
pushfq
pushq $__KERNEL_CS
pushq $repeat_nmi
/* Put stack back */
addq $(6*8), %rsp
nested_nmi_out:
popq %rdx
/* We are returning to kernel mode, so this cannot result in a fault. */
iretq
first_nmi:
/* Restore rdx. */
movq (%rsp), %rdx
/* Make room for "NMI executing". */
pushq $0
/* Leave room for the "iret" frame */
subq $(5*8), %rsp
/* Copy the "original" frame to the "outermost" frame */
.rept 5
pushq 11*8(%rsp)
.endr
UNWIND_HINT_IRET_REGS
/* Everything up to here is safe from nested NMIs */
#ifdef CONFIG_DEBUG_ENTRY
/*
* For ease of testing, unmask NMIs right away. Disabled by
* default because IRET is very expensive.
*/
pushq $0 /* SS */
pushq %rsp /* RSP (minus 8 because of the previous push) */
addq $8, (%rsp) /* Fix up RSP */
pushfq /* RFLAGS */
pushq $__KERNEL_CS /* CS */
pushq $1f /* RIP */
iretq /* continues at repeat_nmi below */
UNWIND_HINT_IRET_REGS
1:
#endif
repeat_nmi:
/*
* If there was a nested NMI, the first NMI's iret will return
* here. But NMIs are still enabled and we can take another
* nested NMI. The nested NMI checks the interrupted RIP to see
* if it is between repeat_nmi and end_repeat_nmi, and if so
* it will just return, as we are about to repeat an NMI anyway.
* This makes it safe to copy to the stack frame that a nested
* NMI will update.
*
* RSP is pointing to "outermost RIP". gsbase is unknown, but, if
* we're repeating an NMI, gsbase has the same value that it had on
* the first iteration. paranoid_entry will load the kernel
* gsbase if needed before we call exc_nmi(). "NMI executing"
* is zero.
*/
movq $1, 10*8(%rsp) /* Set "NMI executing". */
/*
* Copy the "outermost" frame to the "iret" frame. NMIs that nest
* here must not modify the "iret" frame while we're writing to
* it or it will end up containing garbage.
*/
addq $(10*8), %rsp
.rept 5
pushq -6*8(%rsp)
.endr
subq $(5*8), %rsp
end_repeat_nmi:
/*
* Everything below this point can be preempted by a nested NMI.
* If this happens, then the inner NMI will change the "iret"
* frame to point back to repeat_nmi.
*/
pushq $-1 /* ORIG_RAX: no syscall to restart */
/*
* Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
* as we should not be calling schedule in NMI context.
* Even with normal interrupts enabled. An NMI should not be
* setting NEED_RESCHED or anything that normal interrupts and
* exceptions might do.
*/
call paranoid_entry
UNWIND_HINT_REGS
movq %rsp, %rdi
movq $-1, %rsi
call exc_nmi
/* Always restore stashed SPEC_CTRL value (see paranoid_entry) */
IBRS_EXIT save_reg=%r15
/* Always restore stashed CR3 value (see paranoid_entry) */
RESTORE_CR3 scratch_reg=%r15 save_reg=%r14
/*
* The above invocation of paranoid_entry stored the GSBASE
* related information in R/EBX depending on the availability
* of FSGSBASE.
*
* If FSGSBASE is enabled, restore the saved GSBASE value
* unconditionally, otherwise take the conditional SWAPGS path.
*/
ALTERNATIVE "jmp nmi_no_fsgsbase", "", X86_FEATURE_FSGSBASE
wrgsbase %rbx
jmp nmi_restore
nmi_no_fsgsbase:
/* EBX == 0 -> invoke SWAPGS */
testl %ebx, %ebx
jnz nmi_restore
nmi_swapgs:
swapgs
nmi_restore:
POP_REGS
/*
* Skip orig_ax and the "outermost" frame to point RSP at the "iret"
* at the "iret" frame.
*/
addq $6*8, %rsp
/*
* Clear "NMI executing". Set DF first so that we can easily
* distinguish the remaining code between here and IRET from
* the SYSCALL entry and exit paths.
*
* We arguably should just inspect RIP instead, but I (Andy) wrote
* this code when I had the misapprehension that Xen PV supported
* NMIs, and Xen PV would break that approach.
*/
std
movq $0, 5*8(%rsp) /* clear "NMI executing" */
/*
* Skip CLEAR_CPU_BUFFERS here, since it only helps in rare cases like
* NMI in kernel after user state is restored. For an unprivileged user
* these conditions are hard to meet.
*/
/*
* iretq reads the "iret" frame and exits the NMI stack in a
* single instruction. We are returning to kernel mode, so this
* cannot result in a fault. Similarly, we don't need to worry
* about espfix64 on the way back to kernel mode.
*/
iretq
SYM_CODE_END(asm_exc_nmi)
#ifndef CONFIG_IA32_EMULATION
/*
* This handles SYSCALL from 32-bit code. There is no way to program
* MSRs to fully disable 32-bit SYSCALL.
*/
SYM_CODE_START(ignore_sysret)
UNWIND_HINT_EMPTY
mov $-ENOSYS, %eax
CLEAR_CPU_BUFFERS
sysretl
SYM_CODE_END(ignore_sysret)
#endif
.pushsection .text, "ax"
SYM_CODE_START(rewind_stack_and_make_dead)
UNWIND_HINT_FUNC
/* Prevent any naive code from trying to unwind to our caller. */
xorl %ebp, %ebp
movq PER_CPU_VAR(cpu_current_top_of_stack), %rax
leaq -PTREGS_SIZE(%rax), %rsp
UNWIND_HINT_REGS
call make_task_dead
SYM_CODE_END(rewind_stack_and_make_dead)
.popsection
/*
* This sequence executes branches in order to remove user branch information
* from the branch history tracker in the Branch Predictor, therefore removing
* user influence on subsequent BTB lookups.
*
* It should be used on parts prior to Alder Lake. Newer parts should use the
* BHI_DIS_S hardware control instead. If a pre-Alder Lake part is being
* virtualized on newer hardware the VMM should protect against BHI attacks by
* setting BHI_DIS_S for the guests.
*
* CALLs/RETs are necessary to prevent Loop Stream Detector(LSD) from engaging
* and not clearing the branch history. The call tree looks like:
*
* call 1
* call 2
* call 2
* call 2
* call 2
* call 2
* ret
* ret
* ret
* ret
* ret
* ret
*
* This means that the stack is non-constant and ORC can't unwind it with %rsp
* alone. Therefore we unconditionally set up the frame pointer, which allows
* ORC to unwind properly.
*
* The alignment is for performance and not for safety, and may be safely
* refactored in the future if needed.
*/
SYM_FUNC_START(clear_bhb_loop)
push %rbp
mov %rsp, %rbp
movl $5, %ecx
ANNOTATE_INTRA_FUNCTION_CALL
call 1f
jmp 5f
.align 64, 0xcc
ANNOTATE_INTRA_FUNCTION_CALL
1: call 2f
RET
.align 64, 0xcc
2: movl $5, %eax
3: jmp 4f
nop
4: sub $1, %eax
jnz 3b
sub $1, %ecx
jnz 1b
RET
5: lfence
pop %rbp
RET
SYM_FUNC_END(clear_bhb_loop)
EXPORT_SYMBOL_GPL(clear_bhb_loop)
STACK_FRAME_NON_STANDARD(clear_bhb_loop)
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