linux-kernel - Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Date:   Tue, 24 Oct 2017 13:18:59 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Eduardo Valentin <eduval@...zon.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Jan H . Schoenherr" <jschoenh@...zon.de>,
        Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

2017-10-23 17:44-0700, Eduardo Valentin:
> Currently, the existing qspinlock implementation will fallback to
> test-and-set if the hypervisor has not set the PV_UNHALT flag.

Where have you detected the main source of overhead with pinned VCPUs?
Makes me wonder if we couldn't improve general PV_UNHALT,

thanks.

> This patch gives the opportunity to guest kernels to select
> between test-and-set and the regular queueu fair lock implementation
> based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
> flag is not set, the code will still fall back to test-and-set,
> but when the PV_DEDICATED flag is set, the code will use
> the regular queue spinlock implementation.

Some flag makes sense and we do want to make sure that userspaces don't
enable it in pass-through-cpuid mode.