Re: [Qemu-devel] [PATCH v4 5/5] kvm/x86: Hyper-V kvm exit

[Andrey Smetanin]

On 12/21/2015 04:28 PM, Pavel Fedin wrote:
>   Hello!
>
> > Yes, we can use  KVM_EXIT_REG_IO/MSR_IO for Hyper-V SynIC MSRS's changes
> > and can even use only one MSR value . So union inside struct
> > kvm_hyperv_exit is excessive.
> >
> > But we still need Vcpu exit to handle VMBus hypercalls by QEMU to
> > emulate VMBus devices inside QEMU.
> >
> > And currently we are going to extend struct kvm_hyperv_exit
> > to store Hyper-V VMBus hypercall parameters.
>
>   Hm... Hypercalls, you say?
>   We already have KVM_EXIT_HYPERCALL. Documentation says it's currently unused. 
> Is it a leftover from ia64 KVM? Could we reuse it for
> the purpose?
>
> > but could we replace Hyper-V VMBus hypercall and it's parameters
> > by KVM_EXIT_REG_IO/MSR_IO too?
>
>   It depends. Can i read about these hypercalls somewhere? Is there any 
> documentation?
I don't know about a documentation, but you can look at the code of 
Hyper-V hypercall handling inside KVM:

https://github.com/torvalds/linux/blob/master/arch/x86/kvm/hyperv.c#L346

The code simply decodes hypercall parameters from vcpu registers then 
handle hypercall code in switch and encode return code inside vcpu 
registers. Probably encode and decode of hypercall parameters/return 
code can be done in QEMU so we need only some exit with parameter that 
this is Hyper-V hypercall and probably KVM_EXIT_HYPERCALL is good for it.

But KVM_EXIT_HYPERCALL is not used inside KVM/QEMU so requires
implementation.

> Kind regards,
> Pavel Fedin
> Expert Engineer
> Samsung Electronics Research center Russia