linux-kernel - Re: [patch -v2] flat: fix data sections alignment

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Date:	Wed, 4 Mar 2009 20:33:53 +0100
From:	Johannes Weiner <jw@...ix.com>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Russell King <rmk@....linux.org.uk>,
	Bryan Wu <cooloney@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Greg Ungerer <gerg@...inux.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch -v2] flat: fix data sections alignment

On Wed, Mar 04, 2009 at 01:04:00PM -0500, Mike Frysinger wrote:
> On Wed, Mar 4, 2009 at 08:51, Johannes Weiner wrote:
> > The flat loader uses an architecture's flat_stack_align() to align the
> > stack but assumes word-alignment is enough for the data sections.
> >
> > However, on the Xtensa S6000 we have registers up to 128bit width
> > which can be used from userspace and therefor need userspace stack and
> > data-section alignment of at least this size.
> 
> could this perhaps be a gcc problem ?  x86 has a similar problem with
> sse and they addressed it with a function attribute.  after all, just
> because your stack started out 128bit aligned doesnt mean gcc will
> keep it that way when calling other functions.  so having the stack
> start out aligned would only "fix" the stack for the application's
> entry point right (which would in practice bubble up to main()) ?  so
> you'd be right back where you started ...

gcc generates sp changes only ever in multiples of 16 deltas, I just
checked it again with various amounts of stack variables.

The stack frames allocate themselves with an ENTRY instruction and the
generated code I read here allocates stack frames of n * 16 bytes.

So we are good to go as long as the initial stack frame is properly
aligned.

> -mike

	Hannes
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