Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV
 spinlock
To: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
        Oscar Salvador <osalvador@suse.de>, Ingo Molnar <mingo@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org,
        Waiman Long <longman@redhat.com>
References: <20170904142836.15446-1-osalvador@suse.de>
 <20170904144011.gp7hpis6usjehbuf@hirez.programming.kicks-ass.net>
 <20170904222157.GD17982@linux-80c1.suse>
 <0869e8a5-4abd-8f7f-0135-aab3e72e2d01@suse.com>
 <20170905065837.rs767a4os2aumg7h@hirez.programming.kicks-ass.net>
 <924fec17-548a-083d-edce-7adcb662c513@suse.com>
 <20170905081001.hn2276qrhfyqpjdi@hirez.programming.kicks-ass.net>
From: Juergen Gross <jgross@suse.com>
Message-ID: <83ac209b-0807-0a72-cd07-d4ccd1d1ed61@suse.com>
Date: Tue, 5 Sep 2017 10:14:21 +0200
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On 05/09/17 10:10, Peter Zijlstra wrote:
> On Tue, Sep 05, 2017 at 09:35:40AM +0200, Juergen Gross wrote:
>>> So the problem with qspinlock is that it will revert to a classic
>>> test-and-set spinlock if you don't do paravirt but are running a HV.
>>
>> In the Xen case we just use the bare metal settings when xen_nopvspin
>> has been specified. So paravirt, but without modifying any pv_lock_ops
>> functions.
> 
> See arch/x86/include/asm/qspinlock.h:virt_spin_lock(). Unless you clear
> X86_FEATURE_HYPERVISOR you get a test-and-set spinlock.
> 
> And as the comment there says, this is a fallback for !paravirt enabled
> hypervisors to avoid the worst of the lock holder preemption crud.
> 
> But this very much does not deal with the 1:1 case nicely.
> 

Aah, now I've got it.

So maybe we should add virt_spin_lock() to pv_lock_ops? This way e.g.
xen_nopvspin could tweak just the virt_spin_lock() case by letting it
return false all the time?

In case you agree I can setup a patch...


Juergen