Messages in this thread | | | Subject | Re: [PATCH 2/3] jump label: move jump table to r/w section | From | Steven Rostedt <> | Date | Tue, 23 Nov 2010 19:04:09 -0500 |
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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
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