Re: [PATCH/RFC] KVM: do early exit in kvm_check_request

Christian Borntraeger
2016-09-09T18:10:53Z

On 09/09/2016 06:35 PM, Paolo Bonzini wrote:
>
>
> On 09/09/2016 12:10, Christian Borntraeger wrote:
>> By checking vcpu->requests we can do an early exit and allow gcc
>> to optimize multiple kvm_check_request into one block for the
>> common case (no requests).
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>> ---
>> include/linux/kvm_host.h | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index 1c9c973..b15b460 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -1115,6 +1115,8 @@ static inline void kvm_make_request(int req, struct kvm_vcpu *vcpu)
>>
>> static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>> {
>> + if (likely(!vcpu->requests))
>> + return false;
>> if (test_bit(req, &vcpu->requests)) {
>> clear_bit(req, &vcpu->requests);
>
> I'm not sure -- due to asm in test_bit and to -fno-strict-aliasing, I'm
> afraid that each kvm_check_request will have its own zero check.
> kvm_check_request should be rare, but it does show up in microbenchmarks
> so perhaps it's best to keep those two lines of code duplicated across
> the architectures.

Not sure either, it seems to work like above (but does not work when I combine both ifs..)
So yes, maybe its just too unreliable and we keep things as is.