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