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Date:   Thu, 2 Nov 2017 19:24:16 +0100
From:   Paolo Bonzini <>
To:     Eduardo Valentin <>
Cc:, Matt Wilson <>,
        Jonathan Corbet <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
        "H. Peter Anvin" <>,,
        Peter Zijlstra <>,
        Waiman Long <>,,,,
        "Jan H . Schoenherr" <>,
        Anthony Liguori <>
Subject: Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when

On 02/11/2017 19:08, Eduardo Valentin wrote:
> On Thu, Nov 02, 2017 at 06:56:46PM +0100, Paolo Bonzini wrote:
>> On 02/11/2017 18:45, Eduardo Valentin wrote:
>>> Currently, the existing qspinlock implementation will fallback to
>>> test-and-set if the hypervisor has not set the PV_UNHALT flag.
>>> 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.
>> Have you seen Waiman's series that lets you specify this on the guest
>> command line instead?  Would this be acceptable for your use case?
> No, can you please share a link to it? is it already merged to tip/master?

[PATCH-tip v2 0/2] x86/paravirt: Enable users to choose PV lock type

>> (In other words, is there a difference for you between making the host
>> vs. guest administrator toggle the feature?  "" means you are
>> the host admin, how would you use it?)
> The way I think of this is this is a flag set by host side so the
> guest adapts accordingly.
> If the admin in guest side wants to ignore what the host is
> flagging, that is a different story.

Okay, this makes sense.  But perhaps it should be a separate CPUID leaf,
such as "configuration hints", rather than properly a feature.


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