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