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authorJeremy Fitzhardinge <jeremy@xensource.com>2007-07-17 18:37:04 -0700
committerJeremy Fitzhardinge <jeremy@goop.org>2007-07-18 08:47:42 -0700
commit24037a8b69dbf15bfed8fd42a2a2e442d7b0395b (patch)
tree317e7d21bd253200aa09a7b1551d03c1e3985cc0 /arch
parent688340ea34c61ad12473ccd837325b59aada9a93 (diff)
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>
Diffstat (limited to 'arch')
-rw-r--r--arch/i386/kernel/vsyscall-note.S28
1 files changed, 28 insertions, 0 deletions
diff --git a/arch/i386/kernel/vsyscall-note.S b/arch/i386/kernel/vsyscall-note.S
index 52e0cbbac70..271f16a8ca0 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
ELFNOTE_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 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. */
+#define VDSO_NOTE_NONEGSEG_BIT 1
+
+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