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

[Roman Kagan]

On Fri, Dec 18, 2015 at 10:10:11AM -0800, Peter Hornyack wrote:
> On Fri, Dec 18, 2015 at 8:01 AM, Paolo Bonzini <address@hidden> 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.

Roman.