Re: [PATCH 5/8] KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction

From: Sean Christopherson <seanjc@google.com>
To: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5/8] KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction
Date: Wed, 6 Apr 2022 01:48:15 +0000	[thread overview]
Message-ID: <YkzxXw1Aznv4zX0a@google.com> (raw)
In-Reply-To: <7caee33a-da0f-00be-3195-82c3d1cd4cb4@maciej.szmigiero.name>

On Mon, Apr 04, 2022, Maciej S. Szmigiero wrote:
> On 4.04.2022 18:49, Sean Christopherson wrote:
> > On Mon, Apr 04, 2022, Maxim Levitsky wrote:
> > In svm_update_soft_interrupt_rip(), snapshot all information regardless of whether
> > or not nrips is enabled:
> > 
> > 	svm->soft_int_injected = true;
> > 	svm->soft_int_csbase = svm->vmcb->save.cs.base;
> > 	svm->soft_int_old_rip = old_rip;
> > 	svm->soft_int_next_rip = rip;
> > 
> > 	if (nrips)
> > 		kvm_rip_write(vcpu, old_rip);
> > 
> > 	if (static_cpu_has(X86_FEATURE_NRIPS))
> > 		svm->vmcb->control.next_rip = rip;
> > 
> > and then in svm_complete_interrupts(), change the linear RIP matching code to look
> > for the old rip in the nrips case and stuff svm->vmcb->control.next_rip on match.
> > 
> > 	bool soft_int_injected = svm->soft_int_injected;
> > 	unsigned soft_int_rip;
> > 
> > 	svm->soft_int_injected = false;
> > 
> > 	if (soft_int_injected) {
> > 		if (nrips)
> > 			soft_int_rip = svm->soft_int_old_rip;
> > 		else
> > 			soft_int_rip = svm->soft_int_next_rip;
> > 	}
> > 
> > 	...
> > 
> > 	if soft_int_injected && type == SVM_EXITINTINFO_TYPE_EXEPT &&
> > 	   kvm_is_linear_rip(vcpu, soft_int_rip + svm->soft_int_csbase)) {
> > 		if (nrips)
> > 			svm->vmcb->control.next_rip = svm->soft_int_next_rip;
> > 		else
> > 			kvm_rip_write(vcpu, svm->soft_int_old_rip);
> > 	}
> > 
> > 
> > 
> 
> Despite what the svm_update_soft_interrupt_rip() name might suggest this
> handles only *soft exceptions*, not *soft interrupts*
> (which are injected by svm_inject_irq() and also need proper next_rip
> management).

Yeah, soft interrupts are handled in the next patch.  I couldn't come up with a
less awful name.

> Also, I'm not sure that even the proposed updated code above will
> actually restore the L1-requested next_rip correctly on L1 -> L2
> re-injection (will review once the full version is available).

Spoiler alert, it doesn't.  Save yourself the review time.  :-)

The missing piece is stashing away the injected event on nested VMRUN.  Those
events don't get routed through the normal interrupt/exception injection code and
so the next_rip info is lost on the subsequent #NPF.

Treating soft interrupts/exceptions like they were injected by KVM (which they
are, technically) works and doesn't seem too gross.  E.g. when prepping vmcb02

	if (svm->nrips_enabled)
		vmcb02->control.next_rip    = svm->nested.ctl.next_rip;
	else if (boot_cpu_has(X86_FEATURE_NRIPS))
		vmcb02->control.next_rip    = vmcb12_rip;

	if (is_evtinj_soft(vmcb02->control.event_inj)) {
		svm->soft_int_injected = true;
		svm->soft_int_csbase = svm->vmcb->save.cs.base;
		svm->soft_int_old_rip = vmcb12_rip;
		if (svm->nrips_enabled)
			svm->soft_int_next_rip = svm->nested.ctl.next_rip;
		else
			svm->soft_int_next_rip = vmcb12_rip;
	}

And then the VMRUN error path just needs to clear soft_int_injected.