[Trusty][SRU][PATCH] KVM: x86: Check for nested events if there is an injectable interrupt

[Tim Gardner]

On 04/06/2015 08:36 AM, Chris J Arges wrote:
> From: Bandan Das <bsd at redhat.com>
>
> BugLink: http://bugs.launchpad.net/bugs/1413540
>
> With commit b6b8a1451fc40412c57d1 that introduced
> vmx_check_nested_events, checks for injectable interrupts happen
> at different points in time for L1 and L2 that could potentially
> cause a race. The regression occurs because KVM_REQ_EVENT is always
> set when nested_run_pending is set even if there's no pending interrupt.
> Consequently, there could be a small window when check_nested_events
> returns without exiting to L1, but an interrupt comes through soon
> after and it incorrectly, gets injected to L2 by inject_pending_event
> Fix this by adding a call to check for nested events too when a check
> for injectable interrupt returns true
>
> Signed-off-by: Bandan Das <bsd at redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> (cherry picked from commit 9242b5b60df8b13b469bc6b7be08ff6ebb551ad3)
> Signed-off-by: Chris J Arges <chris.j.arges at canonical.com>
> ---
>   arch/x86/kvm/x86.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 677c66a..2515bc8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5834,6 +5834,18 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>   			kvm_x86_ops->set_nmi(vcpu);
>   		}
>   	} else if (kvm_cpu_has_injectable_intr(vcpu)) {
> +		/*
> +		 * Because interrupts can be injected asynchronously, we are
> +		 * calling check_nested_events again here to avoid a race condition.
> +		 * See https://lkml.org/lkml/2014/7/2/60 for discussion about this
> +		 * proposal and current concerns.  Perhaps we should be setting
> +		 * KVM_REQ_EVENT only on certain events and not unconditionally?
> +		 */
> +		if (is_guest_mode(vcpu) && kvm_x86_ops->check_nested_events) {
> +			r = kvm_x86_ops->check_nested_events(vcpu, req_int_win);
> +			if (r != 0)
> +				return r;
> +		}
>   		if (kvm_x86_ops->interrupt_allowed(vcpu)) {
>   			kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu),
>   					    false);
>

Unless I'm missing something in the bug report, there is no information 
there that indicates this patch really does mitigate the soft lockup. It 
looks there are a couple of reporters that would be willing to run a 
test kernel and provide some feedback.

rtg
-- 
Tim Gardner tim.gardner at canonical.com