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    SubjectRe: [patch 17/26] Xen-paravirt_ops: Add nosegneg capability to the vsyscall page notes

    i've Cc:-ed Roland and Ulrich, who should make the call on this one.

    * Jeremy Fitzhardinge <jeremy@goop.org> wrote:

    > Add the "nosegneg" fake capabilty to the vsyscall page notes. This is
    > used by the runtime linker to select a glibc version which then
    > disables negative-offset accesses to the thread-local segment via
    > %gs. These accesses require emulation in Xen (because segments are
    > truncated to protect the hypervisor address space) and avoiding them
    > provides a measurable performance boost.
    >
    > Signed-off-by: Ian Pratt <ian.pratt@xensource.com>
    > Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
    > Signed-off-by: Chris Wright <chrisw@sous-sol.org>
    > Acked-by: Zachary Amsden <zach@vmware.com>
    >
    > ---
    > arch/i386/kernel/vsyscall-note.S | 28 ++++++++++++++++++++++++++++
    > 1 file changed, 28 insertions(+)
    >
    > ===================================================================
    > --- a/arch/i386/kernel/vsyscall-note.S
    > +++ b/arch/i386/kernel/vsyscall-note.S
    > @@ -23,3 +24,31 @@ 3: .balign 4; /* pad out section */
    > ASM_ELF_NOTE_BEGIN(".note.kernel-version", "a", UTS_SYSNAME, 0)
    > .long LINUX_VERSION_CODE
    > ASM_ELF_NOTE_END
    > +
    > +#ifdef CONFIG_XEN
    > +/*
    > + * Add a special note telling glibc's dynamic linker a fake hardware
    > + * flavor that it will use to choose the search path for libraries in the
    > + * same way it uses real hardware capabilities like "mmx".
    > + * We supply "nosegneg" as the fake capability, to indicate that we
    > + * do not like negative offsets in instructions using segment overrides,
    > + * since we implement those inefficiently. This makes it possible to
    > + * install libraries optimized to avoid those access patterns in someplace
    > + * like /lib/i686/tls/nosegneg. Note that an /etc/ld.so.conf.d/file
    > + * corresponding to the bits here is needed to make ldconfig work right.
    > + * It should contain:
    > + * hwcap 0 nosegneg
    > + * to match the mapping of bit to name that we give here.
    > + */
    > +#define NOTE_KERNELCAP_BEGIN(ncaps, mask) \
    > + ASM_ELF_NOTE_BEGIN(".note.kernelcap", "a", "GNU", 2) \
    > + .long ncaps, mask
    > +#define NOTE_KERNELCAP(bit, name) \
    > + .byte bit; .asciz name
    > +#define NOTE_KERNELCAP_END ASM_ELF_NOTE_END
    > +
    > +NOTE_KERNELCAP_BEGIN(1, 1)
    > +NOTE_KERNELCAP(1, "nosegneg")
    > +NOTE_KERNELCAP_END
    > +#endif
    > +
    >
    > --
    >
    > -
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