On Wed, Jun 14, 2017 at 02:11:56PM +0200, Paolo Bonzini wrote: > On 14/06/2017 13:54, Roman Kagan wrote: > >> Why not disable the zeroing for host-initiated MSR writes? This is > >> pretty clearly a KVM bug, we can push it to stable kernels too. > > > > The only problem with this is that QEMU will have no reliable way to > > know if the KVM it runs with has this bug fixed or not. Machines > > without vmbus work and even migrate fine with the current KVM despite > > this bug (the only user of those pages currently is synic timers which > > re-arm themselves and post messages regardless of zeroing). Now > > updating QEMU to a vmbus-enabled version without updating the kernel > > will make the migrations cause guest hangs. > > Return 2 from KVM_CHECK_EXTENSION(KVM_CAP_HYPERV_SYNIC)? Then you can > make new QEMU refuse to enable synic if a new kernel is not available.
Indeed, that's a possibility. I'll probably make it in both directions then: on KVM_ENABLE_CAP(KVM_CAP_HYPERV_SYNIC, 2) disable zeroing completely, including on guest writes, to better match Hyper-V. Or does it deserve a separate cap number? Thanks, Roman.
