Michael Ellerman [off-list ref] writes:

Christophe Leroy [off-list ref] writes:

quoted

__get_datapage() is only a few instructions to retrieve the
address of the page where the kernel stores data to the VDSO.

By inlining this function into its users, a bl/blr pair and
a mflr/mtlr pair is avoided, plus a few reg moves.

The improvement is noticeable (about 55 nsec/call on an 8xx)

vdsotest before the patch:
gettimeofday:    vdso: 731 nsec/call
clock-gettime-realtime-coarse:    vdso: 668 nsec/call
clock-gettime-monotonic-coarse:    vdso: 745 nsec/call

vdsotest after the patch:
gettimeofday:    vdso: 677 nsec/call
clock-gettime-realtime-coarse:    vdso: 613 nsec/call
clock-gettime-monotonic-coarse:    vdso: 690 nsec/call

Signed-off-by: Christophe Leroy <redacted>

This doesn't build with gcc 4.6.3:

  /linux/arch/powerpc/kernel/vdso32/gettimeofday.S: Assembler messages:
  /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:41: Error: unsupported relocation against __kernel_datapage_offset
  /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:86: Error: unsupported relocation against __kernel_datapage_offset
  /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:213: Error: unsupported relocation against __kernel_datapage_offset
  /linux/arch/powerpc/kernel/vdso32/gettimeofday.S:247: Error: unsupported relocation against __kernel_datapage_offset
  make[4]: *** [arch/powerpc/kernel/vdso32/gettimeofday.o] Error 1

Actually I guess it's binutils, which is v2.22 in this case.

Needed this:

diff --git a/arch/powerpc/include/asm/vdso_datapage.h b/arch/powerpc/include/asm/vdso_datapage.h
index 12785f72f17d..0048db347ddf 100644
--- a/arch/powerpc/include/asm/vdso_datapage.h
+++ b/arch/powerpc/include/asm/vdso_datapage.h

@@ -117,7 +117,7 @@ extern struct vdso_data *vdso_data;
 .macro get_datapage ptr, tmp
 	bcl	20, 31, .+4
 	mflr	\ptr
-	addi	\ptr, \ptr, __kernel_datapage_offset - (.-4)
+	addi	\ptr, \ptr, (__kernel_datapage_offset - (.-4))@l
 	lwz	\tmp, 0(\ptr)
 	add	\ptr, \tmp, \ptr
 .endm

cheers