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

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Date:	Tue,  9 Jun 2009 00:52:47 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Petr Tesarik <ptesarik@...e.cz>
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

> > 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.


Thanks,
Roland
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