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@xxxxxxxxxxxxx> > > --- > > 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.
