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

Sean Christopherson
2022-04-06T16:29:13Z

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.