Re: [PATCH 2/2] KVM: x86: Fix split-irqchip vs interrupt injection window request

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 09, 2021, Lai Jiangshan wrote:
> On Fri, Nov 27, 2020 at 7:26 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > kvm_cpu_accept_dm_intr and kvm_vcpu_ready_for_interrupt_injection are
> > a hodge-podge of conditions, hacked together to get something that
> > more or less works.  But what is actually needed is much simpler;
> > in both cases the fundamental question is, do we have a place to stash
> > an interrupt if userspace does KVM_INTERRUPT?
> >
> > In userspace irqchip mode, that is !vcpu->arch.interrupt.injected.
> > Currently kvm_event_needs_reinjection(vcpu) covers it, but it is
> > unnecessarily restrictive.
> >
> > In split irqchip mode it's a bit more complicated, we need to check
> > kvm_apic_accept_pic_intr(vcpu) (the IRQ window exit is basically an INTACK
> > cycle and thus requires ExtINTs not to be masked) as well as
> > !pending_userspace_extint(vcpu).  However, there is no need to
> > check kvm_event_needs_reinjection(vcpu), since split irqchip keeps
> > pending ExtINT state separate from event injection state, and checking
> > kvm_cpu_has_interrupt(vcpu) is wrong too since ExtINT has higher
> > priority than APIC interrupts.  In fact the latter fixes a bug:
> > when userspace requests an IRQ window vmexit, an interrupt in the
> > local APIC can cause kvm_cpu_has_interrupt() to be true and thus
> > kvm_vcpu_ready_for_interrupt_injection() to return false.  When this
> > happens, vcpu_run does not exit to userspace but the interrupt window
> > vmexits keep occurring.  The VM loops without any hope of making progress.
> >
> > Once we try to fix these with something like
> >
> >      return kvm_arch_interrupt_allowed(vcpu) &&
> > -        !kvm_cpu_has_interrupt(vcpu) &&
> > -        !kvm_event_needs_reinjection(vcpu) &&
> > -        kvm_cpu_accept_dm_intr(vcpu);
> > +        (!lapic_in_kernel(vcpu)
> > +         ? !vcpu->arch.interrupt.injected
> > +         : (kvm_apic_accept_pic_intr(vcpu)
> > +            && !pending_userspace_extint(v)));
> >
> > we realize two things.  First, thanks to the previous patch the complex
> > conditional can reuse !kvm_cpu_has_extint(vcpu).  Second, the interrupt
> > window request in vcpu_enter_guest()
> >
> >         bool req_int_win =
> >                 dm_request_for_irq_injection(vcpu) &&
> >                 kvm_cpu_accept_dm_intr(vcpu);
> >
> > should be kept in sync with kvm_vcpu_ready_for_interrupt_injection():
> > it is unnecessary to ask the processor for an interrupt window
> > if we would not be able to return to userspace.  Therefore, the
> > complex conditional is really the correct implementation of
> > kvm_cpu_accept_dm_intr(vcpu).  It all makes sense:
> >
> > - we can accept an interrupt from userspace if there is a place
> >   to stash it (and, for irqchip split, ExtINTs are not masked).
> >   Interrupts from userspace _can_ be accepted even if right now
> >   EFLAGS.IF=0.
> 
> Hello, Paolo
> 
> If userspace does KVM_INTERRUPT, vcpu->arch.interrupt.injected is
> set immediately, and in inject_pending_event(), we have
> 
>         else if (!vcpu->arch.exception.pending) {
>                 if (vcpu->arch.nmi_injected) {
>                         kvm_x86_ops.set_nmi(vcpu);
>                         can_inject = false;
>                 } else if (vcpu->arch.interrupt.injected) {
>                         kvm_x86_ops.set_irq(vcpu);
>                         can_inject = false;
>                 }
>         }
> 
> I'm curious about that can the kvm_x86_ops.set_irq() here be possible
> to queue the irq with EFLAGS.IF=0? If not, which code prevents it?

The interrupt is only directly injected if the local APIC is _not_ in-kernel.
If userspace is managing the local APIC, my understanding is that userspace is
also responsible for honoring EFLAGS.IF, though KVM aids userspace by updating
vcpu->run->ready_for_interrupt_injection when exiting to userspace.  When
userspace is modeling the local APIC, that resolves to
kvm_vcpu_ready_for_interrupt_injection():

	return kvm_arch_interrupt_allowed(vcpu) &&
		kvm_cpu_accept_dm_intr(vcpu);

where kvm_arch_interrupt_allowed() checks EFLAGS.IF (and an edge case related to
nested virtualization).  KVM also captures EFLAGS.IF in vcpu->run->if_flag.
For whatever reason, QEMU checks both vcpu->run flags before injecting an IRQ,
maybe to handle a case where QEMU itself clears EFLAGS.IF?
 
> I'm asking about this because I just noticed that interrupt can
> be queued when exception pending, and this patch relaxed it even
> more.
> 
> Note: interrupt can NOT be queued when exception pending
> until 664f8e26b00c7 ("KVM: X86: Fix loss of exception which
> has not yet been injected") which I think is dangerous.