linux-kernel - Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all paths in skip_emulated

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Message-ID: <20190731235637.GB2845@linux.intel.com>
Date:   Wed, 31 Jul 2019 16:56:37 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all
 paths in skip_emulated_instruction()

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@...el.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);
	}