Re: KVM: x86: better fix for race between nmi injection and enabling nmi window

[Gleb Natapov <gleb@redhat.com>]

On Thu, Mar 31, 2011 at 11:25:46AM +0200, Avi Kivity wrote:
> On 03/31/2011 11:24 AM, Gleb Natapov wrote:
> >On Thu, Mar 31, 2011 at 11:23:28AM +0200, Avi Kivity wrote:
> >>  On 03/30/2011 08:47 PM, Gleb Natapov wrote:
> >>  >On Wed, Mar 30, 2011 at 07:16:34PM +0200, Avi Kivity wrote:
> >>  >>   On 03/30/2011 06:30 PM, Marcelo Tosatti wrote:
> >>  >>   >Based on Gleb's idea, fix race between nmi injection and enabling
> >>  >>   >nmi window in a simpler way.
> >>  >>   >
> >>  >>   >Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
> >>  >>   >
> >>  >>   >diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >>  >>   >index a6a129f..9a7cc1be 100644
> >>  >>   >--- a/arch/x86/kvm/x86.c
> >>  >>   >+++ b/arch/x86/kvm/x86.c
> >>  >>   >@@ -5152,6 +5152,7 @@ static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
> >>  >>   >    static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >>  >>   >    {
> >>  >>   >    	int r;
> >>  >>   >+	int nmi_pending;
> >>  >>   >    	bool req_int_win = !irqchip_in_kernel(vcpu->kvm)&&
> >>  >>   >    		vcpu->run->request_interrupt_window;
> >>  >>   >
> >>  >>   >@@ -5195,11 +5196,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >>  >>   >    	if (unlikely(r))
> >>  >>   >    		goto out;
> >>  >>   >
> >>  >>   >+	nmi_pending = ACCESS_ONCE(vcpu->arch.nmi_pending);
> >>  >>   >+
> >>  >>   >    	if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
> >>  >>   >    		inject_pending_event(vcpu);
> >>  >>   >
> >>  >>   >    		/* enable NMI/IRQ window open exits if needed */
> >>  >>   >-		if (vcpu->arch.nmi_pending)
> >>  >>   >+		if (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);
> >>  >>   >
> >>  >>
> >>  >>   What about the check in inject_pending_events()?
> >>  >>
> >>  >Didn't we decide that this check is not a problem? Worst that can happen
> >>  >is NMI injection will be delayed till next exit.
> >>
> >>  Could be very far in the future.
> >>
> >Next host interrupt. But with tickles host and guest yeah.
> >
> 
> esp. important with NMI, which may be used in a situation where your
> tick (and everything else) are dead.
> 
If host is alive eventually cpu should receive reschedule IPI.

--
			Gleb.