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Message-ID: <20070919112704.GG15500@Krystal>
Date: Wed, 19 Sep 2007 07:27:04 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To: Denys Vlasenko <vda.linux@...glemail.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/7] Immediate Values - Architecture Independent Code
* Denys Vlasenko (vda.linux@...glemail.com) wrote:
> 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
> > declare the section as I do: it will cause no breakage if anybody append
> > data to the structure.
>
> If your structure will be padded by gcc, then this:
>
> +#define immediate_read(name) \
> + ({ \
> + __typeof__(name##__immediate) value; \
> + switch (sizeof(value)) { \
> + case 1: \
> + asm ( ".section __immediate, \"a\", @progbits;\n\t" \
> + ".long %1, (0f)+1, 1;\n\t" \
> + ".previous;\n\t" \
> + "0:\n\t" \
> + "mov %2,%0;\n\t" \
> + : "=r" (value) \
> + : "m" (name##__immediate), \
> + "i" (0)); \
> + break; \
>
> will produce wrongly-sized "struct __immediate" (truncated one),
> since gcc has no idea that you are building struct __immediate there,
> and here:
>
> +void immediate_update_range(const struct __immediate *begin,
> + const struct __immediate *end)
> +{
> + const struct __immediate *iter;
> + int ret;
> +
> + for (iter = begin; iter < end; iter++) {
> + mutex_lock(&immediate_mutex);
> + kernel_text_lock();
> + ret = arch_immediate_update(iter);
> + kernel_text_unlock();
> + if (ret)
> + printk(KERN_WARNING "Invalid immediate value. "
> + "Variable at %p, "
> + "instruction at %p, size %lu\n",
> + (void*)iter->immediate,
> + (void*)iter->var, iter->size);
> + mutex_unlock(&immediate_mutex);
> + }
> +}
>
> iter++ will go off rails.
You are right. It's ok here since we are actually smaller than 32 bytes,
but I should force the structure alignment so that if the structure
grows, the assembly declaration follows. I'll go for the gcc attribute
and then we can remove the section declaration.
Mathieu
> --
> vda
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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