Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all paths in skip_emulated_instruction()

Sean Christopherson
2019-07-31T23:56:41Z

On Wed, Jul 31, 2019 at 04:45:21PM -0700, Jim Mattson wrote:
> On Wed, Jul 31, 2019 at 4:37 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
>
> > 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.
>
> The proposed patch clears the interrupt shadow in the VMCB on all
> paths through svm's skip_emulated_instruction. If this happens at the
> tail end of emulation, it doesn't matter if the full emulator does the
> right thing.

Unless I'm missing something, skip_emulated_instruction() isn't called in
the emulation case, x86_emulate_instruction() updates %rip directly, e.g.:

if (writeback) {
unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
toggle_interruptibility(vcpu, ctxt->interruptibility);
vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
kvm_rip_write(vcpu, ctxt->eip);
if (r == EMULATE_DONE && ctxt->tf)
kvm_vcpu_do_singlestep(vcpu, &r);
if (!ctxt->have_exception ||
exception_type(ctxt->exception.vector) == EXCPT_TRAP)
__kvm_set_rflags(vcpu, ctxt->eflags);

/*
* For STI, interrupts are shadowed; so KVM_REQ_EVENT will
* do nothing, and it will be requested again as soon as
* the shadow expires. But we still need to check here,
* because POPF has no interrupt shadow.
*/
if (unlikely((ctxt->eflags & ~rflags) & X86_EFLAGS_IF))
kvm_make_request(KVM_REQ_EVENT, vcpu);
}