linux-kernel - Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Message-ID: <e190788c-7d90-fa70-a0b1-6352c24624f5@redhat.com>
Date:   Fri, 3 Nov 2017 11:09:53 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eduardo Valentin <eduval@...zon.com>
Cc:     rkrcmar@...hat.com, Matt Wilson <msw@...zon.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: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

On 02/11/2017 19:43, Eduardo Valentin wrote:
> On Thu, Nov 02, 2017 at 07:24:16PM +0100, Paolo Bonzini wrote:
>> 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
>> https://lkml.org/lkml/2017/11/1/655
>>
>>>> (In other words, is there a difference for you between making the host
>>>> vs. guest administrator toggle the feature?  "@amazon.com" 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.
> 
> Oh OK, you don't think this starts to deviate from the feature concept.
> But would the PV_UNHALT also go to "configuration hints" bucket?

PV_UNHALT says whether the pvqspinlock API is available, PV_DEDICATED
says whether it should be used.

> Another way to see this is we have three locking feature options to select from,
> so we need at least two bits here.

PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock
PV_DEDICATED = 0, PV_UNHALT = 0: default is tas

What do you think?

Paolo