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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2014-07-08 07:50, Paolo Bonzini wrote:
> Il 08/07/2014 06:30, Bandan Das ha scritto:
>>
>> 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@redhat.com>
>> ---
>>  arch/x86/kvm/x86.c | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 73537ec..56327a6 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -5907,6 +5907,19 @@ 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)) {
>> +        /*
>> +         * TODO/FIXME: We are calling check_nested_events again
>> +         * here to avoid a race condition. We should really be
>> +         * setting KVM_REQ_EVENT only on certain events
>> +         * and not unconditionally.
>> +         * See https://lkml.org/lkml/2014/7/2/60 for discussion
>> +         * about this proposal and current concerns
>> +         */
>> +        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);
>>
> 
> I think this should be done for NMI as well.

I don't think arch.nmi_pending can flip asynchronously, only in the
context of the VCPU thread - in contrast to pending IRQ states.

> 
> Jan, what do you think?  Can you run Jailhouse through this patch?

Jailhouse seems fine with it, and it resolves the lockup of nested KVM
here as well.

Jan

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