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@xxxxxxxxxxxxx> Signed-off-by: Christian Limpach <Christian.Limpach@xxxxxxxxxxxx> Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx> Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx> Acked-by: Zachary Amsden <zach@xxxxxxxxxx> Cc: Roland McGrath <roland@xxxxxxxxxx> Cc: Ulrich Drepper <drepper@xxxxxxxxxx> --- arch/i386/kernel/vsyscall-note.S | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) =================================================================== --- a/arch/i386/kernel/vsyscall-note.S +++ b/arch/i386/kernel/vsyscall-note.S @@ -12,3 +12,31 @@ ELFNOTE_START(Linux, 0, "a") 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 -- _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
