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. Paolo
