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

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Message-ID: <24e1962a-075c-8168-5d95-033b826ee919@redhat.com>
Date:   Thu, 19 Jul 2018 09:34:58 -0400
From:   Waiman Long <longman@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.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 07/18/2018 09:15 PM, Konrad Rzeszutek Wilk wrote:
> 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?

Right, I will send another patch for Xen. The pvqspinlock code has to be
explicitly opted in. Right now, both Xen and KVM used it in the tree. I
am not sure about other out-of-tree modules. There is nothing I can do
for those.

Cheers,
Longman