[Xen-devel] [patch 02/28]xen: Add nosegneg capability to the vsyscall page notes

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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 |   23 +++++++++++++++++++++++
 include/asm-i386/elf.h           |   14 ++++++++++----
 2 files changed, 33 insertions(+), 4 deletions(-)

===================================================================
--- a/arch/i386/kernel/vsyscall-note.S
+++ b/arch/i386/kernel/vsyscall-note.S
@@ -12,3 +12,26 @@ ELFNOTE_START(Linux, 0, "a")
 ELFNOTE_START(Linux, 0, "a")
        .long LINUX_VERSION_CODE
 ELFNOTE_END
+
+#ifdef CONFIG_XEN
+#include <asm/elf.h>
+
+/*
+ * 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.
+ */
+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
===================================================================
--- a/include/asm-i386/elf.h
+++ b/include/asm-i386/elf.h
@@ -1,10 +1,12 @@
 #ifndef __ASMi386_ELF_H
 #define __ASMi386_ELF_H
 
-/*
- * ELF register definitions..
- */
+/* 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. */
+#define VDSO_NOTE_NONEGSEG_BIT 1
 
+#ifndef __ASSEMBLY__
 #include <asm/ptrace.h>
 #include <asm/user.h>
 #include <asm/auxvec.h>
@@ -24,6 +26,9 @@
 #define R_386_GOTPC    10
 #define R_386_NUM      11
 
+/*
+ * ELF register definitions..
+ */
 typedef unsigned long elf_greg_t;
 
 #define ELF_NGREG (sizeof (struct user_regs_struct) / sizeof(elf_greg_t))
@@ -160,6 +165,7 @@ do if (vdso_enabled) {                                      
                \
                NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);        \
 } while (0)
 
-#endif
+#endif /* ARCH_DLINFO */
+#endif /* __ASSEMBLY__ */
 
 #endif

-- 


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