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[Xen-devel] Re: Performance overhead of paravirt_ops on native identifie

To: Chuck Ebbert <cebbert@xxxxxxxxxx>
Subject: [Xen-devel] Re: Performance overhead of paravirt_ops on native identified
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 21 May 2009 15:48:46 -0700
Cc: Nick Piggin <npiggin@xxxxxxx>, "Xin, Xiaohui" <xiaohui.xin@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, "Li, Xin" <xin.li@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>
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Chuck Ebbert wrote:
On Wed, 13 May 2009 17:16:55 -0700
Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

Paravirt patching turns all the pvops calls into direct calls, so
_spin_lock etc do end up having direct calls.  For example, the compiler
generated code for paravirtualized _spin_lock is:

<_spin_lock+0>:           mov    %gs:0xb4c8,%rax
<_spin_lock+9>:           incl   0xffffffffffffe044(%rax)
<_spin_lock+15>:  callq  *0xffffffff805a5b30
<_spin_lock+22>:  retq

The indirect call will get patched to:
<_spin_lock+0>:           mov    %gs:0xb4c8,%rax
<_spin_lock+9>:           incl   0xffffffffffffe044(%rax)
<_spin_lock+15>:  callq <__ticket_spin_lock>
<_spin_lock+20>:  nop; nop                /* or whatever 2-byte nop */
<_spin_lock+22>:  retq


Can't those calls be changed to jumps?

In this specific instance of this example, yes. But if you start enabling various spinlock debug options then there'll be code following the call. It would be hard for the runtime patching machinery to know when it would be safe to do the substitution.

   J


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