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

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Date:	Tue, 09 Jun 2009 09:57:37 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Roland McGrath <roland@...hat.com>
Cc:	Sam Ravnborg <sam@...nborg.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] x86: clean up vdso-layout.lds.S

Roland McGrath píše v Út 09. 06. 2009 v 00:52 -0700:
> > > I am wondering why we need -P -C here - but we do not need it for lds.S files?
> > > Seems like something we could let go.
> 
> AFAIK these only affect readability of the lds.s output files.
> The difference should be entirely cosmetic.
> 
> > > > >  	.altinstructions	: { *(.altinstructions) }
> > > > > @@ -43,9 +42,49 @@ SECTIONS
> > > > >  	 */
> > > > >  	. = ALIGN(0x100);
> > > 
> > > What is 0x100?
> > 
> > Um. No idea. Roland, you added this line in commit
> > f6b46ebf904f69a73907a5e6b1ed2228e3f03d9e. Can you shed some light on
> > this magic constant?
> 
> 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.
> 
> > What would you expect? The linker script language is quite limited in
> > its capabilities... Best I could do is split the ".broken" section into
> > several sections and move the descriptions from the individual comments
> > above here. If this muckle of empty ".broken.*" sections gets correctly
> > discarded and triggers no bug in binutils, I can probably do it.
> 
> Adding more sections and section names unnecessarily bloats the size of the
> vDSO image.  Keep the set of output sections to the necessary minimum.

Which version of binutils are you using? I'm using binutils-2.19, and
all empty sections get completely discarded, i.e. they do not affect the
size of the output in any way.

Petr Tesarik


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