Commit:     24037a8b69dbf15bfed8fd42a2a2e442d7b0395b
Parent:     688340ea34c61ad12473ccd837325b59aada9a93
Author:     Jeremy Fitzhardinge <[EMAIL PROTECTED]>
AuthorDate: Tue Jul 17 18:37:04 2007 -0700
Committer:  Jeremy Fitzhardinge <[EMAIL PROTECTED]>
CommitDate: Wed Jul 18 08:47:42 2007 -0700

    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 <[EMAIL PROTECTED]>
    Signed-off-by: Christian Limpach <[EMAIL PROTECTED]>
    Signed-off-by: Chris Wright <[EMAIL PROTECTED]>
    Signed-off-by: Jeremy Fitzhardinge <[EMAIL PROTECTED]>
    Acked-by: Zachary Amsden <[EMAIL PROTECTED]>
    Cc: Roland McGrath <[EMAIL PROTECTED]>
    Cc: Ulrich Drepper <[EMAIL PROTECTED]>
 arch/i386/kernel/vsyscall-note.S |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+), 0 deletions(-)

diff --git a/arch/i386/kernel/vsyscall-note.S b/arch/i386/kernel/vsyscall-note.S
index 52e0cbb..271f16a 100644
--- a/arch/i386/kernel/vsyscall-note.S
+++ b/arch/i386/kernel/vsyscall-note.S
@@ -12,3 +12,31 @@
 ELFNOTE_START(Linux, 0, "a")
        .long LINUX_VERSION_CODE
+#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/
+ * corresponding to the bits here is needed to make ldconfig work right.
+ * It should contain:
+ *     hwcap 1 nosegneg
+ * to match the mapping of bit to name that we give here.
+ */
+/* 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; the choice is otherwise arbitrary. */
+       .long 1, 1<<VDSO_NOTE_NONEGSEG_BIT              /* ncaps, mask */
+       .byte VDSO_NOTE_NONEGSEG_BIT; .asciz "nosegneg" /* bit, name */
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