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    SubjectRe: [PATCH 09/30] x86_64: ifdef out struct thread_struct::ip
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    On Thu, 2009-04-09 at 20:53 -0700, Matt Helsley wrote:
    > On Fri, Apr 10, 2009 at 06:35:22AM +0400, Alexey Dobriyan wrote:
    > > struct thread_struct::ip isn't used on x86_64, struct pt_regs::ip is used
    > > instead.
    > >
    > > kgdb should be reading 0, but I can't check it.
    > >
    > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
    > > ---
    > >
    > > arch/x86/include/asm/processor.h | 2 ++
    > > arch/x86/kernel/kgdb.c | 2 +-
    > > 2 files changed, 3 insertions(+), 1 deletion(-)
    > >
    > > --- a/arch/x86/include/asm/processor.h
    > > +++ b/arch/x86/include/asm/processor.h
    > > @@ -421,7 +421,9 @@ struct thread_struct {
    > > unsigned short fsindex;
    > > unsigned short gsindex;
    > > #endif
    > > +#ifdef CONFIG_X86_32
    > > unsigned long ip;
    > > +#endif
    > > #ifdef CONFIG_X86_64
    > > unsigned long fs;
    > > #endif
    >
    > Do these make struct thread_struct behave better in cachelines (smaller,
    > less aliasing)? Can we really fit more in the slab du jour?
    >
    > Otherwise it seems like we're littering these structs with #ifdefs
    > and not really saving anything. If these #ifdefs don't save any space why not
    > just put in a comment:
    >
    > > unsigned long ip; /* Used only on i386 */
    >
    > Or maybe even:
    >
    > union {
    > unsigned long ip; /* Used only on i386 */
    > unsigned long fs; /* Used only on x86_64 */
    > };
    >

    Can we do it like this:
    unsigned long ip_fs; /* ip: i386, fs: x86_64 */

    I am using same variable for both cases, or we can use some better name than ip_fs.
    I am assuming either it is i386 or x86_64 machine ;-)

    --
    JSR




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