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

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 2010-11-23 at 18:55 -0500, Mathieu Desnoyers wrote:
> * Jason Baron (jbaron@redhat.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@redhat.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