linux-kernel - Re: [patch 1/7] Immediate Values - Architecture Independent Code

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Message-ID: <20070918204718.GB12994@Krystal>
Date:	Tue, 18 Sep 2007 16:47:18 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/7] Immediate Values - Architecture Independent Code

* Denys Vlasenko (vda.linux@...glemail.com) wrote:
> On Tuesday 18 September 2007 18:59, Mathieu Desnoyers wrote:
> > * Denys Vlasenko (vda.linux@...glemail.com) wrote:
> > > On Monday 17 September 2007 19:42, Mathieu Desnoyers wrote:
> > > > Index: linux-2.6-lttng/include/asm-generic/vmlinux.lds.h
> > > > ===================================================================
> > > > --- linux-2.6-lttng.orig/include/asm-generic/vmlinux.lds.h	2007-09-17 13:25:06.000000000 -0400
> > > > +++ linux-2.6-lttng/include/asm-generic/vmlinux.lds.h	2007-09-17 13:35:50.000000000 -0400
> > > > @@ -122,6 +122,13 @@
> > > >  		VMLINUX_SYMBOL(__stop___kcrctab_gpl_future) = .;	\
> > > >  	}								\
> > > >  									\
> > > > +	/* Immediate values: pointers */				\
> > > > +	__immediate : AT(ADDR(__immediate) - LOAD_OFFSET) {		\
> > > > +		VMLINUX_SYMBOL(__start___immediate) = .;		\
> > > > +		*(__immediate)						\
> > > > +		VMLINUX_SYMBOL(__stop___immediate) = .;			\
> > > > +	}								\
> > > > +									\
> > > 
> > > Why do you need an output section for that? IOW: will this work too?
> > > 
> > > .data : ... { 
> > > ...
> > > 
> > > 		VMLINUX_SYMBOL(__start___immediate) = .;		\
> > > 		*(__immediate)						\
> > > 		VMLINUX_SYMBOL(__stop___immediate) = .;			\
> > > ...
> > > }
> > > 
> > 
> > This last one could cause alignment problems. We either have to use the
> > proper ALIGN() before the section, or let AT(ADDR(__immediate) -
> > LOAD_OFFSET) take care of it. I prefer the latter.
> 
> This adds yet another output section in vmlinux, and there is
> no tools which need that. We already have 30+ sections there while we need ~20.
> 
> I am trying to fix the mess. Please don't add to it.
> 
> Re alignment: (1) do you really realy REALLY need it? Last I checked,
> i386 was handling unaligned accesses just fine; and
> (2) this works:
> 
> 		. = ALIGN(4)
>  		VMLINUX_SYMBOL(__start___immediate) = .;		\
>  		*(__immediate)						\
>  		VMLINUX_SYMBOL(__stop___immediate) = .;			\
> 
> 

Alignment: I need the __start___immediate and __stop___immediate values
to be at the same alignment as the *(__immediate) content, or else we
end up thinking that padding is data.

. = ALIGN(4) works fine as long as the structure within the section is
not bigger or equal to 32 bytes: gcc has the habit to align 32 bytes
structure on 32 bytes multiples. The safest way I found to do it is to
declare the section as I do: it will cause no breakage if anybody append
data to the structure.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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