powerpc: Add VDSO version of getcpu

We have a request for a fast method of getting CPU and NUMA node IDs
from userspace. This patch implements a getcpu VDSO function,
similar to x86.

Ben suggested we use SPRG3 which is userspace readable. SPRG3 can be
modified by a KVM guest, so we save the SPRG3 value in the paca and
restore it when transitioning from the guest to the host.

I have a glibc patch that implements sched_getcpu on top of this.
Testing on a POWER7:

baseline: 538 cycles
vdso:      30 cycles

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 52c7ad78242e..85b05c463fae 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -533,6 +533,7 @@ int main(void)
         HSTATE_FIELD(HSTATE_VMHANDLER, vmhandler);
         HSTATE_FIELD(HSTATE_SCRATCH0, scratch0);
         HSTATE_FIELD(HSTATE_SCRATCH1, scratch1);
+        HSTATE_FIELD(HSTATE_SPRG3, sprg3);
         HSTATE_FIELD(HSTATE_IN_GUEST, in_guest);
         HSTATE_FIELD(HSTATE_RESTORE_HID5, restore_hid5);
         HSTATE_FIELD(HSTATE_NAPPING, napping);