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powerpc: Add VDSO version of getcpu

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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>
This commit is contained in:
Anton Blanchard
2012-07-04 20:37:11 +00:00
committed by Benjamin Herrenschmidt
parent e6a74c6ea3
commit 18ad51dd34
13 changed files with 140 additions and 4 deletions
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5
arch/powerpc/include/asm/reg.h
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@@ -491,6 +491,7 @@
#define SPRN_SPRG1 0x111 /* Special Purpose Register General 1 */
#define SPRN_SPRG2 0x112 /* Special Purpose Register General 2 */
#define SPRN_SPRG3 0x113 /* Special Purpose Register General 3 */
#define SPRN_USPRG3 0x103 /* SPRG3 userspace read */
#define SPRN_SPRG4 0x114 /* Special Purpose Register General 4 */
#define SPRN_SPRG5 0x115 /* Special Purpose Register General 5 */
#define SPRN_SPRG6 0x116 /* Special Purpose Register General 6 */
@@ -753,14 +754,14 @@
* 64-bit server:
* - SPRG0 unused (reserved for HV on Power4)
* - SPRG2 scratch for exception vectors
* - SPRG3 unused (user visible)
* - SPRG3 CPU and NUMA node for VDSO getcpu (user visible)
* - HSPRG0 stores PACA in HV mode
* - HSPRG1 scratch for "HV" exceptions
*
* 64-bit embedded
* - SPRG0 generic exception scratch
* - SPRG2 TLB exception stack
* - SPRG3 unused (user visible)
* - SPRG3 CPU and NUMA node for VDSO getcpu (user visible)
* - SPRG4 unused (user visible)
* - SPRG6 TLB miss scratch (user visible, sorry !)
* - SPRG7 critical exception scratch