From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 3/4] paravirt: add virt_spin_lock pvops function
Date: Wed, 6 Sep 2017 15:06:00 +0200
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To: Waiman Long <longman@redhat.com>
Cc: Juergen Gross <jgross@suse.com>, jeremy@goop.org, x86@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, chrisw@sous-sol.org, mingo@redhat.com, tglx@linutronix.de, hpa@zytor.com, xen-devel@lists.xenproject.org, akataria@vmware.com, boris.ostrovsky@oracle.com
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On Wed, Sep 06, 2017 at 08:44:09AM -0400, Waiman Long wrote:
> On 09/06/2017 03:08 AM, Peter Zijlstra wrote:
> > Guys, please trim email.
> >
> > On Tue, Sep 05, 2017 at 10:31:46AM -0400, Waiman Long wrote:
> >> For clarification, I was actually asking if you consider just adding one
> >> more jump label to skip it for Xen/KVM instead of making
> >> virt_spin_lock() a pv-op.
> > I don't understand. What performance are you worried about. Native will
> > now do: "xor rax,rax; jnz some_cold_label" that's fairly trival code.
> 
> It is not native that I am talking about. I am worry about VM with
> non-Xen/KVM hypervisor where virt_spin_lock() will actually be called.
> Now that function will become a callee-saved function call instead of
> being inlined into the native slowpath function.

But only if we actually end up using the test-and-set thing, because if
you have paravirt we end up using that.

And the test-and-set thing sucks anyway. But yes, you're right, that
case gets worse.