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From: Andrey Smetanin <asmetanin@virtuozzo.com>
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Date: Mon, 21 Dec 2015 16:48:48 +0300
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Subject: Re: [Qemu-devel] [PATCH v4 5/5] kvm/x86: Hyper-V kvm exit
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To: Pavel Fedin <p.fedin@samsung.com>, 'Roman Kagan' <rkagan@virtuozzo.com>, 'Peter Hornyack' <peterhornyack@google.com>, 'Paolo Bonzini' <pbonzini@redhat.com>, 'kvm list' <kvm@vger.kernel.org>, "'Denis V. Lunev'" <den@openvz.org>, 'Gleb Natapov' <gleb@kernel.org>, qemu-devel@nongnu.org



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
>
>