Messages in this thread | | | Subject | Re: [q] kbuild for private asm-offsets (Re: [PATCH 6/10] lguest code: the little linux hypervisor.) | From | Rusty Russell <> | 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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