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

[Marcelo Tosatti <mtosatti@redhat.com>]

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.

> +
> + 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).