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Commit fc161239 authored by Raghavendra K T's avatar Raghavendra K T Committed by Marcelo Tosatti
Browse files

KVM: Add documentation on hypercalls 



Thanks Alex for KVM_HC_FEATURES inputs and Jan for VAPIC_POLL_IRQ,
and Peter (HPA) for suggesting hypercall ABI addition.

Signed-off-by: default avatarRaghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 51d59c6b

Documentation/virtual/kvm/hypercalls.txt

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Linux KVM Hypercall:

===================

X86:

 KVM Hypercalls have a three-byte sequence of either the vmcall or the vmmcall

 instruction. The hypervisor can replace it with instructions that are

 guaranteed to be supported.



 Up to four arguments may be passed in rbx, rcx, rdx, and rsi respectively.

 The hypercall number should be placed in rax and the return value will be

 placed in rax.  No other registers will be clobbered unless explicitly stated

 by the particular hypercall.



S390:

  R2-R7 are used for parameters 1-6. In addition, R1 is used for hypercall

  number. The return value is written to R2.



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

  number in R1.



 PowerPC:

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

  Return value is placed in R3.



  KVM hypercalls uses 4 byte opcode, that are patched with 'hypercall-instructions'

  property inside the device tree's /hypervisor node.

  For more information refer to Documentation/virtual/kvm/ppc-pv.txt



KVM Hypercalls Documentation

===========================

The template for each hypercall is:

1. Hypercall name.

2. Architecture(s)

3. Status (deprecated, obsolete, active)

4. Purpose



1. KVM_HC_VAPIC_POLL_IRQ

------------------------

Architecture: x86

Status: active

Purpose: Trigger guest exit so that the host can check for pending

interrupts on reentry.



2. KVM_HC_MMU_OP

------------------------

Architecture: x86

Status: deprecated.

Purpose: Support MMU operations such as writing to PTE,

flushing TLB, release PT.



3. KVM_HC_FEATURES

------------------------

Architecture: PPC

Status: active

Purpose: Expose hypercall availability to the guest. On x86 platforms, cpuid

used to enumerate which hypercalls are available. On PPC, either device tree

based lookup ( which is also what EPAPR dictates) OR KVM specific enumeration

mechanism (which is this hypercall) can be used.



4. KVM_HC_PPC_MAP_MAGIC_PAGE

------------------------

Architecture: PPC

Status: active

Purpose: To enable communication between the hypervisor and guest there is a

shared page that contains parts of supervisor visible register state.

The guest can map this shared page to access its supervisor register through

memory using this hypercall.