linux-kernel - Re: [PATCH 2/3] jump label: move jump table to r/w section

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Date:	Tue, 23 Nov 2010 19:04:09 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jason Baron <jbaron@...hat.com>, mingo@...e.hu,
	peterz@...radead.org, hpa@...or.com, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, sam@...nborg.org,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] jump label: move jump table to r/w section

On Tue, 2010-11-23 at 18:55 -0500, Mathieu Desnoyers wrote:
> * Jason Baron (jbaron@...hat.com) wrote:
> > Since we writing the jump table it should be be in R/W kernel
> > section. Move it to DATA_DATA
> > 
> > Signed-off-by: Jason Baron <jbaron@...hat.com>
> > ---
> >  include/asm-generic/vmlinux.lds.h |   14 ++++----------
> >  1 files changed, 4 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index bd69d79..9ca894d 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -161,6 +161,10 @@
> >  	VMLINUX_SYMBOL(__start___tracepoints) = .;			\
> >  	*(__tracepoints)						\
> >  	VMLINUX_SYMBOL(__stop___tracepoints) = .;			\
> > +	. = ALIGN(8);							\
> 
> Past churn with various architectures and compiler with tracepoints,
> markers and immediate values lead me to hint at the following approach
> for jump label structure alignment:
> 
> . = ALIGN(32);
> 
> and to modify jump_label.h to have:
> 
> struct jump_entry {
> 	jump_label_t code;
> 	jump_label_t target;
> 	jump_label_t key;
> } __attribute__((aligned(32)));
> 
> Otherwise, the compiler is free to choose on which value it prefers to
> align the jump_entry structures, which might not match the address at
> which the linker scripts puts the beginning of the jump table.
> 
> In this case, given that we put put the jump label table after the
> tracepoint table, we should be already aligned on 32 bytes. But I would
> recommend to put the . = ALIGN(32) in the linker script anyway, just for
> documentation purpose (and it should not add any padding in this case).
> 
> This is not a problem introduced by this patch, it also applies to the
> current jump label code.

Agreed, but this change could probably wait for 2.6.38.

Also, if this is done, then an it should be wrapped in a
#ifdef CONFIG_JUMP_LABEL and only inserted if we are using jump labels.
Otherwise we may add a 32 byte hole for nothing. I know it's small, but
why waste it if you don't need to.

-- Steve


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