On 22/06/2017 07:14, Roman Kagan wrote: > 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... The new capability should also check that args[0] is zero. That was a mistake done previously that we shouldn't repeat. Paolo
