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

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Message-Id: <200709190945.02556.vda.linux@googlemail.com>
Date:	Wed, 19 Sep 2007 09:45:02 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/7] Immediate Values - Architecture Independent Code

On Tuesday 18 September 2007 21:47, Mathieu Desnoyers wrote:
> * 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

Yes, I'm painfully aware of that. gcc is too damn happy to align stuff.

> declare the section as I do: it will cause no breakage if anybody append
> data to the structure.

You can actively fight gcc's sadistic alignment tendencies instead:

struct __immediate {
       long var;               /* Identifier variable of the immediate value */
       long immediate;         /*
                                * Pointer to the memory location that holds
                                * the immediate value within the load immediate
                                * instruction.
                                */
       long size;              /* Type size. */
} __attribute__ ((aligned(sizeof(long))));  <================= HERE

Kernel is already using this technique a lot. Try

grep -r '^\} *__attribute__ *.*aligned' .
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