Messages in this thread | | | From | Denys Vlasenko <> | Subject | Re: [patch 1/7] Immediate Values - Architecture Independent Code | Date | Wed, 19 Sep 2007 09:45:02 +0100 |
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On Tuesday 18 September 2007 21:47, Mathieu Desnoyers wrote: > * Denys Vlasenko (vda.linux@googlemail.com) wrote: > > On Tuesday 18 September 2007 18:59, Mathieu Desnoyers wrote: > > > * Denys Vlasenko (vda.linux@googlemail.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' . -- vda - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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