linux-kernel - Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Date:   Thu, 9 Nov 2017 18:28:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Radim Krcmar <rkrcmar@...hat.com>
Cc:     Pankaj Gupta <pagupta@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Paolo Bonzini <pbonzini@...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>,
        the arch/x86 maintainers <x86@...nel.org>,
        Waiman Long <longman@...hat.com>, kvm <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: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

On Thu, Nov 09, 2017 at 06:15:11PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 09, 2017 at 06:12:41PM +0100, Peter Zijlstra wrote:
> > On Thu, Nov 09, 2017 at 05:45:23PM +0100, Radim Krcmar wrote:
> > > 2017-11-09 17:17+0100, Peter Zijlstra:
> > > > On Thu, Nov 09, 2017 at 05:05:36PM +0100, Radim Krcmar wrote:
> > > > > 2017-11-09 10:53-0500, Pankaj Gupta:
> > > > > > 2] PV TLB should also behave as per option PV_DEDICATED for better performance.
> > > > > 
> > > > > Right,
> > > > 
> > > > Shouldn't KVM do flush_tlb_other() in any case? Not sure how
> > > > PV_DEDICATED can help with that.
> > > 
> > > It will, the suggestion was based on recent extension of the
> > > flush_tlb_others implementaion, https://lkml.org/lkml/2017/11/8/1146.
> > > 
> > > PV_TLB_FLUSH allows a guest to set a flush bit instead of sending flush
> > > IPI if the target VCPU is not running.  This would be a waste of time
> > > with PV_DEDICATED as all VCPUs are expected to always running.
> > > 
> > > With PV_DEDICATED, the guest should keep using native_flush_tlb_others.
> > 
> > Is saving that for_each_cpu() really worth the effort compared to the
> > cost of actually doing the IPIs and CR3 write?
> > 
> > Also, you should not put cpumask_t on stack, that's 'broken'.
> 
> Also, you'll want to use __cpumask_clear_cpu() there.

Also^2, that patch split is crazy, after patch 2/3 your machine is
broken due to lost TLB flushes. You have to first add the SHOULD_FLUSH
handling and then clear CPUs from the native_flush_tlb_other() mask.