Re: [PATCH v2 1/2] x86:kvm:hyperv: add KVM_CAP_HYPERV_SYNIC2

[Roman Kagan <rkagan@virtuozzo.com>]

On Wed, Jun 21, 2017 at 10:09:15PM +0200, Radim Krčmář wrote:
> 2017-06-16 20:37+0300, Roman Kagan:
> > There is a flaw in the Hyper-V SynIC implementation in KVM: when message
> > page or event flags page is enabled by setting the corresponding msr,
> > KVM zeroes it out.  This is problematic because on migration the
> > corresponding MSRs are loaded on the destination, so the content of
> > those pages is lost.
> > 
> > This went unnoticed so far because the only user of those pages was
> > in-KVM hyperv synic timers, which could continue working despite that
> > zeroing.
> > 
> > Newer QEMU uses those pages for Hyper-V VMBus implementation, and
> > zeroing them breaks the migration.
> > 
> > Besides, in newer QEMU the content of those pages is fully managed by
> > QEMU, so zeroing them is undesirable even when writing the MSRs from the
> > guest side.
> > 
> > To support this new scheme, introduce a new capability,
> > KVM_CAP_HYPERV_SYNIC2, which, when enabled, makes sure that the synic
> > pages aren't zeroed out in KVM.
> > 
> > Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
> > ---
> 
> I have changed the subject tags to more common "kvm: x86: hyperv:" and
> added minimal documentation:
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 4029943887a3..3f4b1d5f0dce 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -4157,3 +4157,10 @@ Currently the following bits are defined for the device_irq_level bitmap:
>  Future versions of kvm may implement additional events. These will get
>  indicated by returning a higher number from KVM_CHECK_EXTENSION and will be
>  listed above.
> +
> +8.10 KVM_CAP_HYPERV_SYNIC2
> +
> +Architectures: x86
> +
> +The only difference from KVM_CAP_HYPERV_SYNIC is that KVM does not clear SynIC
> +pages when the guest enables them.
> 
> Please let me know if you'd like to improve it.

Yes that's perfectly fine, thanks.  Sorry, I should have done it
myself...

Thanks,
Roman.