linux-kernel - Re: [PATCH] x86: clean up vdso-layout.lds.S

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Message-ID: <4A2E82BA.7040905@zytor.com>
Date:	Tue, 09 Jun 2009 08:41:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland McGrath <roland@...hat.com>
CC:	Petr Tesarik <ptesarik@...e.cz>, Sam Ravnborg <sam@...nborg.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] x86: clean up vdso-layout.lds.S

Roland McGrath wrote:
> 
> You mean this one:
> 
> 	/*
> 	 * Align the actual code well away from the non-instruction data.
> 	 * This is the best thing for the I-cache.
> 	 */
> 	. = ALIGN(0x100);
> 
> Reading the comment might make it obvious that it's intended for optimal
> code alignment.  I suspect someone at the time told me 256 is as big as an
> I-cache line was ever likely to get.  You could use L1_CACHE_BYTES instead
> I suppose.
> 

Most likely 256 was chosen as a compromise between the the
then-documented value for coherency avoidance (128), future-proofing,
and waste.

I don't think we want to use different values on different platforms,
and end up with dramatically different vdsos when they still need to fit
in the same size envelope.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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