linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Date:	Thu, 22 Jan 2009 14:55:07 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	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>,
	"hpa@...or.com" <hpa@...or.com>,
	"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

On Thu, 2009-01-22 at 14:44 -0800, 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.

What about removing the identity functions entirely.  They are useless,
really.  All that is needed is a patch site filled with nops for Xen to
overwrite, just stuffing the value into the proper registers.  For
64-bit, it can be a simple mov to satisfy the constraints.

> Also, I just posted patches to get rid of all pvops calls when fetching 
> or setting flags in a pte, which I hope will help.

Sounds like it will help.

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