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Subject: Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all
 paths in skip_emulated_instruction()
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Jim Mattson <jmattson@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Joerg Roedel <joro@8bytes.org>
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 <20190731235637.GB2845@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Thu, 1 Aug 2019 02:13:38 +0200
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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);
> 	}
>