120 lines
4.1 KiB
ReStructuredText
120 lines
4.1 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=============================
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The s390 DIAGNOSE call on KVM
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=============================
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KVM on s390 supports the DIAGNOSE call for making hypercalls, both for
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native hypercalls and for selected hypercalls found on other s390
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hypervisors.
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Note that bits are numbered as by the usual s390 convention (most significant
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bit on the left).
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General remarks
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---------------
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DIAGNOSE calls by the guest cause a mandatory intercept. This implies
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all supported DIAGNOSE calls need to be handled by either KVM or its
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userspace.
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All DIAGNOSE calls supported by KVM use the RS-a format::
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--------------------------------------
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| '83' | R1 | R3 | B2 | D2 |
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--------------------------------------
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0 8 12 16 20 31
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The second-operand address (obtained by the base/displacement calculation)
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is not used to address data. Instead, bits 48-63 of this address specify
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the function code, and bits 0-47 are ignored.
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The supported DIAGNOSE function codes vary by the userspace used. For
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DIAGNOSE function codes not specific to KVM, please refer to the
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documentation for the s390 hypervisors defining them.
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DIAGNOSE function code 'X'500' - KVM virtio functions
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-----------------------------------------------------
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If the function code specifies 0x500, various virtio-related functions
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are performed.
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General register 1 contains the virtio subfunction code. Supported
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virtio subfunctions depend on KVM's userspace. Generally, userspace
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provides either s390-virtio (subcodes 0-2) or virtio-ccw (subcode 3).
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Upon completion of the DIAGNOSE instruction, general register 2 contains
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the function's return code, which is either a return code or a subcode
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specific value.
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Subcode 0 - s390-virtio notification and early console printk
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Handled by userspace.
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Subcode 1 - s390-virtio reset
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Handled by userspace.
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Subcode 2 - s390-virtio set status
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Handled by userspace.
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Subcode 3 - virtio-ccw notification
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Handled by either userspace or KVM (ioeventfd case).
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General register 2 contains a subchannel-identification word denoting
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the subchannel of the virtio-ccw proxy device to be notified.
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General register 3 contains the number of the virtqueue to be notified.
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General register 4 contains a 64bit identifier for KVM usage (the
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kvm_io_bus cookie). If general register 4 does not contain a valid
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identifier, it is ignored.
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After completion of the DIAGNOSE call, general register 2 may contain
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a 64bit identifier (in the kvm_io_bus cookie case), or a negative
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error value, if an internal error occurred.
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See also the virtio standard for a discussion of this hypercall.
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DIAGNOSE function code 'X'501 - KVM breakpoint
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----------------------------------------------
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If the function code specifies 0x501, breakpoint functions may be performed.
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This function code is handled by userspace.
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This diagnose function code has no subfunctions and uses no parameters.
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DIAGNOSE function code 'X'9C - Voluntary Time Slice Yield
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---------------------------------------------------------
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General register 1 contains the target CPU address.
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In a guest of a hypervisor like LPAR, KVM or z/VM using shared host CPUs,
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DIAGNOSE with function code 0x9c may improve system performance by
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yielding the host CPU on which the guest CPU is running to be assigned
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to another guest CPU, preferably the logical CPU containing the specified
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target CPU.
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DIAG 'X'9C forwarding
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+++++++++++++++++++++
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The guest may send a DIAGNOSE 0x9c in order to yield to a certain
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other vcpu. An example is a Linux guest that tries to yield to the vcpu
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that is currently holding a spinlock, but not running.
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However, on the host the real cpu backing the vcpu may itself not be
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running.
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Forwarding the DIAGNOSE 0x9c initially sent by the guest to yield to
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the backing cpu will hopefully cause that cpu, and thus subsequently
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the guest's vcpu, to be scheduled.
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diag9c_forwarding_hz
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KVM kernel parameter allowing to specify the maximum number of DIAGNOSE
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0x9c forwarding per second in the purpose of avoiding a DIAGNOSE 0x9c
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forwarding storm.
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A value of 0 turns the forwarding off.
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