Commit 24037a8b authored by Jeremy Fitzhardinge's avatar Jeremy Fitzhardinge Committed by Jeremy Fitzhardinge
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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: default avatarIan Pratt <>
Signed-off-by: default avatarChristian Limpach <>
Signed-off-by: default avatarChris Wright <>
Signed-off-by: default avatarJeremy Fitzhardinge <>
Acked-by: default avatarZachary Amsden <>
Cc: Roland McGrath <>
Cc: Ulrich Drepper <>
parent 688340ea
......@@ -12,3 +12,31 @@
ELFNOTE_START(Linux, 0, "a")
* 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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