xen: disable vdso "nosegneg" on native boot

One of the nice ideas behind paravirt is that CONFIG_XEN=y can be included
in a standard configuration and be no worse for native booting than as a
Xen guest.  The glibc feature that supports the vDSO "nosegneg" note is
designed specifically to make this easy.  You just have to flip one bit at
boot time.  This patch makes Xen flip the bit, so a CONFIG_XEN=y kernel on
bare hardware does not make glibc use the less-optimized library builds.

Signed-off-by: Roland McGrath <roland@redhat.com>
Acked-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 6 deletions

diff --git a/arch/i386/kernel/vsyscall-note.S b/arch/i386/kernel/vsyscall-note.S
index 271f16a8ca01..07c0daf78237 100644
--- a/arch/i386/kernel/vsyscall-note.S
+++ b/arch/i386/kernel/vsyscall-note.S
@@ -14,7 +14,6 @@ ELFNOTE_START(Linux, 0, "a")
 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
@@ -28,15 +27,19 @@ ELFNOTE_END
  * It should contain:
  *      hwcap 1 nosegneg
  * to match the mapping of bit to name that we give here.
+ *
+ * At runtime, the fake hardware feature will be considered to be present
+ * if its bit is set in the mask word.  So, we start with the mask 0, and
+ * at boot time we set VDSO_NOTE_NONEGSEG_BIT if running under Xen.
  */
 
-/* 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
+#include "../xen/vdso.h"        /* Defines VDSO_NOTE_NONEGSEG_BIT.  */
 
+        .globl VDSO_NOTE_MASK
 ELFNOTE_START(GNU, 2, "a")
-        .long 1, 1<<VDSO_NOTE_NONEGSEG_BIT              /* ncaps, mask */
+        .long 1                 /* ncaps */
+VDSO_NOTE_MASK:
+        .long 0                 /* mask */
         .byte VDSO_NOTE_NONEGSEG_BIT; .asciz "nosegneg" /* bit, name */
 ELFNOTE_END
 #endif