Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>> 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.
Interesting. I don’t think that this test_bit usually goes through the asm
part (since usually req is constant). However, it seems that due to the
indirection to vcpu->requests, the compiler does not use a register to cache
vcpu->requests. It is shameful in places like vcpu_enter_guest. Perhaps
checking the bit directly in kvm_check_request is wiser?
Nadav
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