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

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Message-ID: <20101123235519.GB21549@Krystal>
Date:	Tue, 23 Nov 2010 18:55:19 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Jason Baron <jbaron@...hat.com>
Cc:	rostedt@...dmis.org, 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

* 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.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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