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, 4 insertions, 0 deletions

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index e764e2361d47..5a84c8d3d040 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -1064,6 +1064,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_206)
         mtspr   SPRN_DABR,r5
         mtspr   SPRN_DABRX,r6
 
+        /* Restore SPRG3 */
+        ld      r3,HSTATE_SPRG3(r13)
+        mtspr   SPRN_SPRG3,r3
+
         /*
          * Reload DEC.  HDEC interrupts were disabled when
          * we reloaded the host's LPCR value.