Re: [PATCH] Documentation/kvm : Add documentation on Hypercalls

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Aug 01, 2012 at 04:19:01PM +0530, Raghavendra K T wrote:
> On 08/01/2012 08:37 AM, Marcelo Tosatti wrote:
> >On Tue, Jul 24, 2012 at 02:23:59PM +0530, Raghavendra K T wrote:
> >>From: Raghavendra K T<raghavendra.kt@linux.vnet.ibm.com>
> >>
> >>Thanks Alex for KVM_HC_FEATURES inputs and Jan for VAPIC_POLL_IRQ,
> >>and Peter (HPA) for suggesting hypercall ABI addition.
> >>
> >>Signed-off-by: Raghavendra K T<raghavendra.kt@linux.vnet.ibm.com>
> >>---
> >>Please have a closer look at Hypercall ABI newly added
> >>
> >>Changes since last post:
> >>  - Added hypercall ABI (Peter)
> >>  - made KVM_HC_VAPIC_POLL_IRQ active explicitly (Randy)
> >>
> >>TODO: We need to add history details of each hypercall as suggested by HPA,
> >>which I could not trace easily. Hope it is easy for hypercall authors
> >>
> >>  Documentation/virtual/hypercalls.txt |   71 ++++++++++++++++++++++++++++++++++
> >>  1 files changed, 71 insertions(+), 0 deletions(-)
> >>  create mode 100644 Documentation/virtual/hypercalls.txt
> >>
> >>diff --git a/Documentation/virtual/hypercalls.txt b/Documentation/virtual/hypercalls.txt
> >>new file mode 100644
> >>index 0000000..caffc08
> >>--- /dev/null
> >>+++ b/Documentation/virtual/hypercalls.txt
> >>@@ -0,0 +1,71 @@
> >>+Hypercall ABI:
> >>+=============
> >>+A brief look at calling conventions of X86, S390 and PPC
> >>+X86:
> >>+ KVM Hypercalls have a three-byte sequence of either the vmrun or the vmmrun
> >>+ instruction. The hypervisor can replace it with instructions that are
> >>+ guaranteed to be supported.
> >
> >vmcall.
> 
> How is this:
>  KVM Hypercalls have a three-byte sequence of vmcall instruction. But
> The hypervisor can replace it with the instructions that are guaranteed
> to be supported.
> 
> (Related question: Is it not that vmcall instruction is patched by 0f
> 01 09 vmmcall instruction sequence for amd?. 

Yes, it is.

Linux guest binary contains the Intel vmcall instruction, which is
patched on AMD to vmmcall.

> So I think we need to
> change documentation I referred from arch/x86/include/asm/kvm_para.h.
> It should be vmcall / vmmcall instead of vmrun and vmmrun)

Yes.

> >>+
> >>+ 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.
> >
> >It depends on the hypercall. It happens that current hypercalls use
> >the four registers, but its not an ABI (hyper-v hypercalls uses r8, for example).
> >
> 
> Okay, agree. I did not know about hyper-v. Above one is for X86 and
> KVM  (and not X86 / hyper-v). So I should remove ABI and probably say,

Just say "Linux x86 Hypercall", yeah.

> KVM x86 Hypercall
> ==================
> 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.
> 
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