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

To:	Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject:	[Xen-devel] Re: Performance overhead of paravirt_ops on native identified
From:	Chuck Ebbert <cebbert@xxxxxxxxxx>
Date:	Thu, 21 May 2009 18:42:33 -0400
Cc:	Nick Piggin <npiggin@xxxxxxx>, "Xin, Xiaohui" <xiaohui.xin@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Linux, 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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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?

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