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

[Andrey Smetanin <asmetanin@virtuozzo.com>]

On 12/18/2015 09:39 PM, Roman Kagan wrote:
> On Fri, Dec 18, 2015 at 10:10:11AM -0800, Peter Hornyack wrote:
>> On Fri, Dec 18, 2015 at 8:01 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>> On 18/12/2015 16:19, Pavel Fedin wrote:
>>>> As far as i understand this code, KVM_EXIT_HYPERV is called when one
>>>> of three MSRs are accessed. But, shouldn't we have implemented
>>>> instead something more generic, like KVM_EXIT_REG_IO, which would
>>>> work similar to KVM_EXIT_PIO or KVM_EXIT_MMIO, but carry register
>>>> code and value?
>>>
>>> Yes, we considered that.  There were actually patches for this as well.
>>>   However, in this case the register is still emulated in the kernel, and
>>> userspace just gets informed of the new value.
>>
>> On brief inspection of Andrey's patch (I have not been following
>> closely) it looks like the kvm_hyperv_exit struct that's returned to
>> userspace contains more data (control, evt_page, and msg_page fields)
>> than simply the value of the MSR, so would the desired SynIC exit fit
>> into a general-purpose exit for MSR emulation?
>
> Frankly I'm struggling trying to recall why we implemented it this way.
> Actually all three fields are the values of respective MSRs and I don't
> see any necessity to pass all three at the same time when any of them
> gets updated.  The patch for QEMU adds an exit handler which processes
> the fields individually, so I have a strong suspicion that union was
> meant here rather than struct.
>
> I hope Andrey will help to shed some light on that when he's back in the
> office on Monday; meanwhile I think this peculiarity can be ignored.
Hello!

We have implemented Hyper-V related Vcpu exit not only for Hyper-V SynIC 
MSR's changes but also to provide future interface to transfer guest 
VMBus hypercalls parameters into QEMU.

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.

SynIC MSR's changes could be replaced by KVM_EXIT_REG_IO/MSR_IO
but could we replace Hyper-V VMBus hypercall and it's parameters
by KVM_EXIT_REG_IO/MSR_IO too?

>
> Roman.
>