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>

arch/i386/boot/compressed/relocs.c

@@ -31,6 +31,7 @@ static const char* safe_abs_relocs[] = {
 		"__kernel_rt_sigreturn",
 		"__kernel_sigreturn",
 		"SYSENTER_RETURN",
+		"VDSO_NOTE_MASK",
 		"xen_irq_disable_direct_reloc",
 		"xen_save_fl_direct_reloc",
 };

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

arch/i386/xen/setup.c

@@ -19,6 +19,7 @@
 #include <xen/features.h>
 
 #include "xen-ops.h"
+#include "vdso.h"
 
 /* These are code, but not functions.  Defined in entry.S */
 extern const char xen_hypervisor_callback[];
@@ -55,6 +56,18 @@ static void xen_idle(void)
 	}
 }
 
+/*
+ * Set the bit indicating "nosegneg" library variants should be used.
+ */
+static void fiddle_vdso(void)
+{
+	extern u32 VDSO_NOTE_MASK; /* See ../kernel/vsyscall-note.S.  */
+	extern char vsyscall_int80_start;
+	u32 *mask = (u32 *) ((unsigned long) &VDSO_NOTE_MASK - VDSO_PRELINK +
+			     &vsyscall_int80_start);
+	*mask |= 1 << VDSO_NOTE_NONEGSEG_BIT;
+}
+
 void __init xen_arch_setup(void)
 {
 	struct physdev_set_iopl set_iopl;
@@ -93,4 +106,6 @@ void __init xen_arch_setup(void)
 #endif
 
 	paravirt_disable_iospace();
+
+	fiddle_vdso();
 }

arch/i386/xen/vdso.h

@@ -0,0 +1,4 @@
+/* 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