Re: [PATCH 1/6] KVM: Check for pending events before attempting injection

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jul 28, 2010 at 07:31:03PM +0300, Avi Kivity wrote:
>  On 07/28/2010 07:21 PM, Marcelo Tosatti wrote:
> >On Tue, Jul 27, 2010 at 04:19:35PM +0300, Avi Kivity wrote:
> >>Instead of blindly attempting to inject an event before each guest entry,
> >>check for a possible event first in vcpu->requests.  Sites that can trigger
> >>event injection are modified to set KVM_REQ_EVENT:
> >>
> >>- interrupt, nmi window opening
> >>- ppr updates
> >>- i8259 output changes
> >>- local apic irr changes
> >>- rflags updates
> >>- gif flag set
> >>- event set on exit
> >>
> >>This improves non-injecting entry performance, and sets the stage for
> >>non-atomic injection.
> >>
> >>Signed-off-by: Avi Kivity<avi@redhat.com>
> >>---
> >>  arch/x86/kvm/i8259.c     |    1 +
> >>  arch/x86/kvm/lapic.c     |   12 ++++++++++--
> >>  arch/x86/kvm/svm.c       |    8 +++++++-
> >>  arch/x86/kvm/vmx.c       |    6 ++++++
> >>  arch/x86/kvm/x86.c       |   35 ++++++++++++++++++++++++++---------
> >>  include/linux/kvm_host.h |    1 +
> >>  6 files changed, 51 insertions(+), 12 deletions(-)
> >>
> >>@@ -4731,17 +4737,19 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >>  		goto out;
> >>  	}
> >>
> >>-	inject_pending_event(vcpu);
> >>+	if (kvm_check_request(KVM_REQ_EVENT, vcpu)) {
> >>+		inject_pending_event(vcpu);
> >>
> >>-	/* enable NMI/IRQ window open exits if needed */
> >>-	if (vcpu->arch.nmi_pending)
> >>-		kvm_x86_ops->enable_nmi_window(vcpu);
> >>-	else if (kvm_cpu_has_interrupt(vcpu) || req_int_win)
> >>-		kvm_x86_ops->enable_irq_window(vcpu);
> >>+		/* enable NMI/IRQ window open exits if needed */
> >>+		if (vcpu->arch.nmi_pending)
> >>+			kvm_x86_ops->enable_nmi_window(vcpu);
> >>+		else if (kvm_cpu_has_interrupt(vcpu) || req_int_win)
> >>+			kvm_x86_ops->enable_irq_window(vcpu);
> >Problem is it might not be possible to inject the event signalled by
> >KVM_REQ_EVENT, say an interrupt from an irqchip, if there is an event
> >that needs reinjection (or an exception).
> 
> That can happen event now, no?  A pending exception, interrupt comes
> along, injection picks up the exception but leaves the interrupt.
> 
> Now the situation can be more complicated:
> 
> - pending exception
> - injection
> - interrupt, sets KVM_REQ_EVENT
> - notices KVM_REQ_EVENT
> - drops KVM_REQ_EVENT, cancels exception (made pending again)
> - goes back
> - injection (injects exception again, interrupt is pending)
> 
> as far as I can tell, this is all fine.

But you cleared KVM_REQ_EVENT. Which means you're not going to inject
the pending interrupt on the next entry.