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Date: Wed, 15 Nov 2017 20:54:07 -0800
From: Eduardo Valentin <eduval@amazon.com>
To: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
CC: Eduardo Valentin <eduval@amazon.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Matt Wilson <msw@amazon.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Thomas Gleixner" <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Waiman Long <longman@redhat.com>, <kvm@vger.kernel.org>,
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        "Jan H . Schoenherr" <jschoenh@amazon.de>,
        Anthony Liguori <aliguori@amazon.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
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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.


>  	/* 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