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Date: Tue, 5 Sep 2017 10:28:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Juergen Gross <jgross@suse.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
        Oscar Salvador <osalvador@suse.de>, Ingo Molnar <mingo@kernel.org>,
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        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV
 spinlock
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On Tue, Sep 05, 2017 at 10:14:21AM +0200, Juergen Gross wrote:
> 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?

Hmm, that might work. Could we somehow nop that call when
!X86_FEATURE_HYPERVISOR?, that saves native from having to do the call
and would be a win for everyone.