KVM: s390: diagnose call documentation

  S390 uses diagnose instruction as hypercall (0x500) along with hypercall

  number in R1.

  For further information on the S390 diagnose call as supported by KVM,

  refer to Documentation/virtual/kvm/s390-diag.txt.

 PowerPC:

  It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.

  Return value is placed in R3.

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).

    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.