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

Paolo Bonzini
2016-09-09T16:35:52Z

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.

Paolo