On 06/06/2017 20:19, Roman Kagan wrote: > There is a design 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 violates the spec in general (per spec, > the pages have to be overlay ones and only zeroed at cpu reset), but > it's non-fatal in normal operation because the user exit happens after > the page is zeroed, so it's the underlying guest page which is zeroed > out, and sane guests don't depend on its contents to be preserved while > it's overlaid. > > However, in the case of vmstate load the overlay pages are set up before > msrs are set so the contents of those pages get lost. > > To work it around, avoid setting up overlay pages in .post_load. > Instead, postpone it until after the msrs are pushed to KVM. As a > result, KVM just zeroes out the underlying guest pages similar to how it > happens during guest-initiated msr writes, which is tolerable.
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. Paolo
