linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Message-Id: <1232665120.16317.186.camel@bodhitayantram.eng.vmware.com>
Date:	Thu, 22 Jan 2009 14:58:40 -0800
From:	Zachary Amsden <zach@...are.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

On Thu, 2009-01-22 at 14:49 -0800, H. Peter Anvin wrote:

> 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.

Yes, for known trivial ops (most!), there isn't any reason to ever have
a call to begin with; simply an inline instruction sequence would be
fine, and only those callers that override the sequence would need to
patch.  It's possible to write clever macros to assure there is always
space for a 5 byte call.

Zach

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