On Wed, Jul 31, 2019 at 01:27:53PM -0700, Jim Mattson wrote: > On Wed, Jul 31, 2019 at 9:37 AM Paolo Bonzini <[email protected]> wrote: > > > > On 31/07/19 15:50, Vitaly Kuznetsov wrote: > > > Jim Mattson <[email protected]> writes: > > > > > >> On Thu, Jun 20, 2019 at 4:02 AM Vitaly Kuznetsov <[email protected]> > > >> 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.)
At a glance, the full emulator models behavior correctly, e.g. see toggle_interruptibility() and setters of ctxt->interruptibility. I'm pretty sure that leaves the EPT misconfig MMIO and APIC access EOI fast paths as the only (VMX) path that would incorrectly handle a MOV/POP SS. Reading the guest's instruction stream to detect MOV/POP SS would defeat the whole "fast path" thing, not to mention both paths aren't exactly architecturally compliant in the first place.
