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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 <>
Signed-off-by: Christian Limpach <>
Signed-off-by: Chris Wright <>
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)
+#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 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
+NOTE_KERNELCAP(1, "nosegneg")
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