Subject: Re: [q] kbuild for private asm-offsets (Re: [PATCH 6/10] lguest
	code: the little linux hypervisor.)
From: Rusty Russell <rusty@rustcorp.com.au>
To: Oleg Verych <olecom@flower.upol.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>, Andi Kleen <ak@muc.de>,
       virtualization@lists.osdl.org,
       lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
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Date: Tue, 13 Feb 2007 10:41:36 +1100
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On Mon, 2007-02-12 at 14:34 +0100, Oleg Verych wrote:
> > I'd like my own, private "asm-offsets.h".  In this case, in
> > arch/i386/lguest/.  I guess it's a matter of extracting the core of the
> > asm-offsets.h magic and generalizing it.
> >
> > Have a good break!
> > Rusty.
> 
> If you will have time for newbie, to explain in a few words, what is it need
> for (whole idea, or key detail), and, maybe, why it is generated so ... interestingly:
> 
>             asm-offsets.c -> *.s -> *.h
>  (but this looks like interconnecting C and assembler, obviously)

Hi Oleg,

	Always happy to explain.  There's often a need to access constants in
assembler, which can only be derived from C, such as the size of a
structure, or the offset of a certain member within a structure.
Hardcoding the numbers in assembler is fragile leading to breakage when
something changes.

	So, asm-offsets.c is the solution: it uses asm() statements to emit
patterns in the assembler, with the compiler computing the actual
numbers, eg:

	#define DEFINE(sym, val) \
	        asm volatile("\n->" #sym " %0 " #val : : "i" (val))
	DEFINE(SIZEOF_FOOBAR, sizeof(foobar));

Becomes in asm-offsets.s:
	->SIZEOF_FOOBAR $10 sizeof(foobar)  #

This gets sed'd back into asm-offsets.h:
	#define SIZEOF_FOOBAR 10 /* SIZEOF_FOOBAR  # */

This can be included from .S files (which get passed through the
pre-processor).

Hope that helps!
Rusty.

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