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    Subject[RFC PATCH 27/35] Add nosegneg capability to the vsyscall page notes
    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>
    ---
    arch/i386/kernel/vsyscall-note.S | 29 +++++++++++++++++++++++++++++
    1 files changed, 29 insertions(+)

    --- xen-subarch-2.6.orig/arch/i386/kernel/vsyscall-note.S
    +++ xen-subarch-2.6/arch/i386/kernel/vsyscall-note.S
    @@ -3,6 +3,7 @@
    * Here we can supply some information useful to userland.
    */

    +#include <linux/config.h>
    #include <linux/uts.h>
    #include <linux/version.h>

    @@ -23,3 +24,31 @@
    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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