Commit 24037a8b authored by Jeremy Fitzhardinge's avatar Jeremy Fitzhardinge Committed by Jeremy Fitzhardinge

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 <ian.pratt@xensource.com>
Signed-off-by: default avatarChristian Limpach <Christian.Limpach@cl.cam.ac.uk>
Signed-off-by: default avatarChris Wright <chrisw@sous-sol.org>
Signed-off-by: default avatarJeremy Fitzhardinge <jeremy@xensource.com>
Acked-by: default avatarZachary Amsden <zach@vmware.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ulrich Drepper <drepper@redhat.com>
parent 688340ea
...@@ -12,3 +12,31 @@ ...@@ -12,3 +12,31 @@
ELFNOTE_START(Linux, 0, "a") ELFNOTE_START(Linux, 0, "a")
.long LINUX_VERSION_CODE .long LINUX_VERSION_CODE
ELFNOTE_END 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
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment