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SubjectRe: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set
Hi Eduardo,
2017-11-16 12:54 GMT+08:00 Eduardo Valentin <eduval@amazon.com>:
> Hey Radim,
>
> On Thu, Nov 09, 2017 at 03:17:33PM +0100, Radim Krčmář wrote:
>
> <cut>
>
>>
>> This is what I'm doubting, because the patch is adding about two
>> thousand cycles to every spinlock-taken path.
>> Doesn't this patch yield better results?
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 3df743b60c80..d9225e48c11a 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -676,6 +676,12 @@ void __init kvm_spinlock_init(void)
>> {
>> if (!kvm_para_available())
>> return;
>> +
>> + if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) {
>> + static_branch_disable(&virt_spin_lock_key);
>> + return;
>> + }
>> +
>
> Yes, the above suggestion is a much better approach. The code has probably changed from the time I wrote the first version. I will refresh with the above suggestion.

Do you mind to send a new version since the merge window is closed?

Regards,
Wanpeng Li

>
>
>> /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>> if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>> return;
>>
>> > However, the key aspect
>> > here is this patch gives a way for the host to instruct the guest to use qspinlock.
>> > Even with Longman's patch which allows guest to select the spinlock implementation,
>> > there should still be the auto-select mode. In such mode, PV_DEDICATED should
>> > allow the host to get the guest to use qspinlock, without, the guest will fallback
>> > to tas when PV_UNHALT == 0.
>>
>> I agree that a flag can be useful for certains setups.
>
> Cool!
>
>>
>
> --
> All the best,
> Eduardo Valentin

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