Re: [Qemu-devel] [PATCH 16/23] hyperv: map overlay pages after updating msrs

[Roman Kagan]

On Wed, Jun 14, 2017 at 02:11:56PM +0200, Paolo Bonzini wrote:
> On 14/06/2017 13:54, Roman Kagan wrote:
> >> Why not disable the zeroing for host-initiated MSR writes?  This is
> >> pretty clearly a KVM bug, we can push it to stable kernels too.
> >
> > The only problem with this is that QEMU will have no reliable way to
> > know if the KVM it runs with has this bug fixed or not.  Machines
> > without vmbus work and even migrate fine with the current KVM despite
> > this bug (the only user of those pages currently is synic timers which
> > re-arm themselves and post messages regardless of zeroing).  Now
> > updating QEMU to a vmbus-enabled version without updating the kernel
> > will make the migrations cause guest hangs.
> 
> Return 2 from KVM_CHECK_EXTENSION(KVM_CAP_HYPERV_SYNIC)?  Then you can
> make new QEMU refuse to enable synic if a new kernel is not available.

Indeed, that's a possibility.

I'll probably make it in both directions then: on
KVM_ENABLE_CAP(KVM_CAP_HYPERV_SYNIC, 2) disable zeroing completely,
including on guest writes, to better match Hyper-V.  Or does it deserve
a separate cap number?

Thanks,
Roman.