linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Message-ID: <4978F7DC.1040503@zytor.com>
Date:	Thu, 22 Jan 2009 14:49:00 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Zachary Amsden <zach@...are.com>, Nick Piggin <npiggin@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"jeremy@...source.com" <jeremy@...source.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

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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