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.
