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

[Paolo Bonzini]

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.

Paolo