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>,
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>
Subject: Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock
Date: Tue, 5 Sep 2017 10:28:01 +0200 [thread overview]
Message-ID: <20170905082801.d3sr7hdjz6lpon5p@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <83ac209b-0807-0a72-cd07-d4ccd1d1ed61@suse.com>
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.
next prev parent reply other threads:[~2017-09-05 8:28 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-04 14:28 [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock Oscar Salvador
2017-09-04 14:40 ` Peter Zijlstra
2017-09-04 19:32 ` Waiman Long
2017-09-04 22:21 ` Davidlohr Bueso
2017-09-05 6:28 ` Juergen Gross
2017-09-05 6:57 ` Oscar Salvador
2017-09-05 8:07 ` Peter Zijlstra
2017-09-05 6:58 ` Peter Zijlstra
2017-09-05 7:35 ` Juergen Gross
2017-09-05 8:10 ` Peter Zijlstra
2017-09-05 8:14 ` Juergen Gross
2017-09-05 8:28 ` Peter Zijlstra [this message]
2017-09-05 8:52 ` Juergen Gross
2017-09-05 9:03 ` Peter Zijlstra
2017-09-05 9:11 ` Juergen Gross
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