linux-kernel - Re: [PATCH] x86/pvqspinlock: Use LOCK_PREFIX in __pv_queued_spin

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Date:   Wed, 18 Jul 2018 09:28:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Will Deacon <will.deacon@....com>,
        Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH] x86/pvqspinlock: Use LOCK_PREFIX in
 __pv_queued_spin_unlock assembly code

On 07/18/2018 04:19 AM, Peter Zijlstra wrote:
> On Tue, Jul 17, 2018 at 04:16:00PM -0400, Waiman Long wrote:
>> The LOCK_PREFIX macro should be used in the assembly
>> __raw_callee_save___pv_queued_spin_unlock code so that the lock prefix
>> can be patched out on UP systems.
> True; but does paravirt locking realy make sense on UP guests?
>
Yes, it doesn't make sense. That is why I have another kvm patch that
turns off pvqspinlock for UP guest. However, I am not that familiar with
the Xen code. So I don't have a corresponding one for Xen.

Cheers,
Longman