On 08/14/2012 07:13 PM, Alexander Graf wrote:
>
> On 15.08.2012, at 02:10, Scott Wood wrote:
>
>> On 08/14/2012 06:04 PM, Alexander Graf wrote:
>>> We need a central place to check for pending requests in. Add one that
>>> only does the timer check we already do in a different place.
>>>
>>> Later, this central function can be extended by more checks.
>>>
>>> Signed-off-by: Alexander Graf <agraf@xxxxxxx>
>>> ---
>>> arch/powerpc/kvm/booke.c | 24 +++++++++++++++++-------
>>> 1 files changed, 17 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
>>> index 1d4ce9a..bcf87fe 100644
>>> --- a/arch/powerpc/kvm/booke.c
>>> +++ b/arch/powerpc/kvm/booke.c
>>> @@ -419,13 +419,6 @@ static void kvmppc_core_check_exceptions(struct kvm_vcpu *vcpu)
>>> unsigned long *pending = &vcpu->arch.pending_exceptions;
>>> unsigned int priority;
>>>
>>> - if (vcpu->requests) {
>>> - if (kvm_check_request(KVM_REQ_PENDING_TIMER, vcpu)) {
>>> - smp_mb();
>>> - update_timer_ints(vcpu);
>>> - }
>>> - }
>>> -
>>> priority = __ffs(*pending);
>>> while (priority < BOOKE_IRQPRIO_MAX) {
>>> if (kvmppc_booke_irqprio_deliver(vcpu, priority))
>>> @@ -461,6 +454,14 @@ int kvmppc_core_prepare_to_enter(struct kvm_vcpu *vcpu)
>>> return r;
>>> }
>>>
>>> +static void kvmppc_check_requests(struct kvm_vcpu *vcpu)
>>> +{
>>> + if (vcpu->requests) {
>>> + if (kvm_check_request(KVM_REQ_PENDING_TIMER, vcpu))
>>> + update_timer_ints(vcpu);
>>> + }
>>> +}
>>> +
>>> /*
>>> * Common checks before entering the guest world. Call with interrupts
>>> * disabled.
>>> @@ -485,6 +486,15 @@ static int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
>>> break;
>>> }
>>>
>>> + smp_mb();
>>> + if (vcpu->requests) {
>>> + /* Make sure we process requests preemptable */
>>> + local_irq_enable();
>>> + kvmppc_check_requests(vcpu);
>>> + local_irq_disable();
>>> + continue;
>>> + }
>>
>> What previous memory access is the smp_mb() ordering against?
>
> Against itself really (see the continue?). I might have just gotten this wrong though :).
Ordering is already ensured by the hardware in that case since it's the
same address -- and the stuff inside the if statement is more than
enough to ensure the compiler doesn't cache an old value of vcpu->requests.
That said, I think the problem is not that the smp_mb() shouldn't be
there, but that there should be a vcpu->mode setting before it.
-Scott
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