On Tuesday 18 September 2007 21:47, Mathieu Desnoyers wrote:
> * Denys Vlasenko ([email protected]) wrote:
> > On Tuesday 18 September 2007 18:59, Mathieu Desnoyers wrote:
> > > * Denys Vlasenko ([email protected]) 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
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