[patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall page notes
From: Jeremy Fitzhardinge <hidden>
Date: 2007-02-16 04:46:34
Also in:
lkml, xen-devel
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 <redacted> Signed-off-by: Christian Limpach <redacted> Signed-off-by: Chris Wright <redacted> --- arch/i386/kernel/vsyscall-note.S | 28 +++++++++++++++++++++++++++++ 1 files changed, 28 insertions(+) ===================================================================
--- a/arch/i386/kernel/vsyscall-note.S
+++ b/arch/i386/kernel/vsyscall-note.S@@ -23,3 +24,31 @@ 3: .balign 4; /* pad out section */ ASM_ELF_NOTE_BEGIN(".note.kernel-version", "a", UTS_SYSNAME, 0) .long LINUX_VERSION_CODE ASM_ELF_NOTE_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 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 +#define NOTE_KERNELCAP_END ASM_ELF_NOTE_END + +NOTE_KERNELCAP_BEGIN(1, 1) +NOTE_KERNELCAP(1, "nosegneg") +NOTE_KERNELCAP_END +#endif +
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