linux-kernel - Re: [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

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Message-ID: <20180719011546.GB28585@char.US.ORACLE.com>
Date:   Wed, 18 Jul 2018 21:15:46 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

On Tue, Jul 17, 2018 at 05:59:27PM -0400, Waiman Long wrote:
> On a VM with only 1 vCPU, the locking fast path will always be
> successful. In this case, there is no need to use the the PV qspinlock
> code which has higher overhead on the unlock side than the native
> qspinlock code.

Why not make this global? That is for both KVM and Xen and any
other virtualized guest that uses this?

> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  arch/x86/kernel/kvm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 5b2300b..575c9a5 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -748,6 +748,10 @@ void __init kvm_spinlock_init(void)
>  	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
>  		return;
>  
> +	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
> +	if (num_possible_cpus() == 1)
> +		return;
> +
>  	__pv_init_lock_hash();
>  	pv_lock_ops.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
>  	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
> -- 
> 1.8.3.1
>