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    Subject[patch 02/28]xen: 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>
    Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
    Acked-by: Zachary Amsden <zach@vmware.com>
    Cc: Roland McGrath <roland@redhat.com>
    Cc: Ulrich Drepper <drepper@redhat.com>

    ---
    arch/i386/kernel/vsyscall-note.S | 23 +++++++++++++++++++++++
    include/asm-i386/elf.h | 14 ++++++++++----
    2 files changed, 33 insertions(+), 4 deletions(-)

    ===================================================================
    --- a/arch/i386/kernel/vsyscall-note.S
    +++ b/arch/i386/kernel/vsyscall-note.S
    @@ -12,3 +12,26 @@ ELFNOTE_START(Linux, 0, "a")
    ELFNOTE_START(Linux, 0, "a")
    .long LINUX_VERSION_CODE
    ELFNOTE_END
    +
    +#ifdef CONFIG_XEN
    +#include <asm/elf.h>
    +
    +/*
    + * 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.
    + */
    +ELFNOTE_START(GNU, 2, "a")
    + .long 1, 1<<VDSO_NOTE_NONEGSEG_BIT /* ncaps, mask */
    + .byte VDSO_NOTE_NONEGSEG_BIT; .asciz "nosegneg" /* bit, name */
    +ELFNOTE_END
    +#endif
    ===================================================================
    --- a/include/asm-i386/elf.h
    +++ b/include/asm-i386/elf.h
    @@ -1,10 +1,12 @@
    #ifndef __ASMi386_ELF_H
    #define __ASMi386_ELF_H

    -/*
    - * ELF register definitions..
    - */
    +/* Bit used for the pseudo-hwcap for non-negative segments. We use
    + bit 1 to avoid bugs in some versions of glibc when bit 0 is
    + used. */
    +#define VDSO_NOTE_NONEGSEG_BIT 1

    +#ifndef __ASSEMBLY__
    #include <asm/ptrace.h>
    #include <asm/user.h>
    #include <asm/auxvec.h>
    @@ -24,6 +26,9 @@
    #define R_386_GOTPC 10
    #define R_386_NUM 11

    +/*
    + * ELF register definitions..
    + */
    typedef unsigned long elf_greg_t;

    #define ELF_NGREG (sizeof (struct user_regs_struct) / sizeof(elf_greg_t))
    @@ -160,6 +165,7 @@ do if (vdso_enabled) { \
    NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE); \
    } while (0)

    -#endif
    +#endif /* ARCH_DLINFO */
    +#endif /* __ASSEMBLY__ */

    #endif
    --

    -
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