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

[Chris J Arges]

On 04/06/2015 10:17 AM, Tim Gardner wrote:
> 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,
I can reproduce this issue at will, and have done the testing to show
this reduces the occurrences of softlockups. In addition, the patch
itsself references a patch we already have on the 3.13 distro kernel
(b6b8a1451fc40412c57d1).

--chris