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

["Radim Krčmář" <rkrcmar@redhat.com>]

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.