linux-kernel - Re: Performance overhead of paravirt

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Message-ID: <20090521184233.3c3e97ad@dhcp-100-2-144.bos.redhat.com>
Date:	Thu, 21 May 2009 18:42:33 -0400
From:	Chuck Ebbert <cebbert@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	"Li, Xin" <xin.li@...el.com>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: Performance overhead of paravirt_ops on native identified

On Wed, 13 May 2009 17:16:55 -0700
Jeremy Fitzhardinge <jeremy@...p.org> 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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