Message-ID: <4978F7DC.1040503@zytor.com>
Date: Thu, 22 Jan 2009 14:49:00 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Zachary Amsden <zach@vmware.com>, Nick Piggin <npiggin@suse.de>,
       Ingo Molnar <mingo@elte.hu>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       "jeremy@xensource.com" <jeremy@xensource.com>,
       "chrisw@sous-sol.org" <chrisw@sous-sol.org>,
       "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>,
       Andrew Morton <akpm@linux-foundation.org>,
       Xen-devel <xen-devel@lists.xensource.com>
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT
References: <20090120110542.GE19505@wotan.suse.de>	 <20090120112634.GA20858@elte.hu> <20090120140324.GA26424@elte.hu>	 <49763806.5090009@goop.org> <20090120205653.GA19710@elte.hu>	 <20090121072718.GN24891@wotan.suse.de>  <4977A051.8050203@goop.org> <1232663311.16317.176.camel@bodhitayantram.eng.vmware.com> <4978F6C6.3090003@goop.org>
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Jeremy Fitzhardinge wrote:
> 
> I did have some patches to do that at one point.  If you set pgd_val = 
> paravirt_nop, then the patching machinery will completely nop out the 
> call site.  The problem is that it depends on the calling convention 
> using the same regs for the first arg and return - true for 32-bit, but 
> not 64.  We could fix that with identity functions which the patcher 
> recognizes and can replace with either pure nops or inline appropriate 
> register moves.
> 

There is also the option to use assembly wrappers to avoid relying on 
the calling convention.  This is particularly so since we have sites 
where as little as a two-byte instruction gets bloated up with huge 
push/pop sequences around a tiny instruction.  Those would be better 
served with a direct call to a stub (5 bytes), which would be repatched 
to the two-byte instruction + 3 byte nop.

	-hpa

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