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

Paolo Bonzini
2019-08-01T00:13:44Z

On 01/08/19 01:56, Sean Christopherson wrote:
> 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.:

Indeed. skip_emulated_instruction() is only used when the vmexit code
takes care of emulation directly.

Paolo

> 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);
> }
>