On Wed, Jul 31, 2019 at 9:37 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 31/07/19 15:50, Vitaly Kuznetsov wrote: > > Jim Mattson <jmattson@xxxxxxxxxx> writes: > > > >> On Thu, Jun 20, 2019 at 4:02 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > >>> > >>> Regardless of the way how we skip instruction, interrupt shadow needs to be > >>> cleared. > >> > >> This change is definitely an improvement, but the existing code seems > >> to assume that we never call skip_emulated_instruction on a > >> POP-SS/MOV-to-SS/STI. Is that enforced anywhere? > > > > (before I send v1 of the series) I looked at the current code and I > > don't think it is enforced, however, VMX version does the same and > > honestly I can't think of a situation when we would be doing 'skip' for > > such an instruction.... and there's nothing we can easily enforce from > > skip_emulated_instruction() as we have no idea what the instruction > > is... Can't we still coerce kvm into emulating any instruction by leveraging a stale ITLB entry? The 'emulator' kvm-unit-test did this before the KVM forced emulation prefix was introduced, but I haven't checked to see if the original (admittedly fragile) approach still works. Also, for POP-SS, you could always force emulation by mapping the %rsp address beyond guest physical memory. The hypervisor would then have to emulate the instruction to provide bus-error semantics. > I agree, I think a comment is worthwhile but we can live with the > limitation. I think we can live with the limitation, but I'd really prefer to see a KVM exit with KVM_INTERNAL_ERROR_EMULATION for an instruction that kvm doesn't emulate properly. That seems better than just a comment that the virtual CPU doesn't behave as architected. (I realize that I am probably in the minority here.)
