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Date: Thu, 2 Nov 2017 11:27:38 -0700
From: Eduardo Valentin <eduval@amazon.com>
To: Waiman Long <longman@redhat.com>
CC: Eduardo Valentin <eduval@amazon.com>,
        Paolo Bonzini <pbonzini@redhat.com>, <rkrcmar@redhat.com>,
        Matt Wilson <msw@amazon.com>, Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, <x86@kernel.org>,
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        "Jan H . Schoenherr" <jschoenh@amazon.de>,
        Anthony Liguori <aliguori@amazon.com>
Subject: Re: [PATCHv2 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
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Longman,

On Thu, Nov 02, 2017 at 02:12:13PM -0400, Waiman Long wrote:
> On 11/02/2017 02:08 PM, 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?
> 
> See https://lkml.org/lkml/2017/11/1/655

Oh I see, thanks. I think that patch would help, but I believe the series and this patch are complementary.

Paolo, back to your question, I think this patch still makes sense in combination with Waiman's series
for the following case:

+ * If this argument is not specified, the kernel will automatically choose
+ * an appropriate one depending on X86_FEATURE_HYPERVISOR and hypervisor
+ * specific settings.
+ */

 In this case, the hypervisor can still flag PV_DEDICATED and the guest would not pick test&set, when
that scenario is desirable.

> 
> Cheers,
> Longman
> 

-- 
All the best,
Eduardo Valentin