powerpc/32: Add VDSO version of getcpu on non SMP

Commit 18ad51dd ("powerpc: Add VDSO version of getcpu") added
getcpu() for PPC64 only, by making use of a user readable general
purpose SPR.

PPC32 doesn't have any such SPR.

For non SMP, just return CPU id 0 from the VDSO directly.
PPC32 doesn't support CONFIG_NUMA so NUMA node is always 0.

Before the patch, vdsotest reported:
getcpu: syscall: 1572 nsec/call
getcpu:    libc: 1787 nsec/call
getcpu:    vdso: not tested

Now, vdsotest reports:
getcpu: syscall: 1582 nsec/call
getcpu:    libc: 502 nsec/call
getcpu:    vdso: 187 nsec/call
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eaac4b6494ecff1811220fccc895bf282aab884a.1575273217.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/vdso32/Makefile

@@ -2,9 +2,7 @@
 
 # List of files in the vdso, has to be asm only for now
 
-obj-vdso32-$(CONFIG_PPC64) = getcpu.o
-obj-vdso32 = sigtramp.o gettimeofday.o datapage.o cacheflush.o note.o \
-		$(obj-vdso32-y)
+obj-vdso32 = sigtramp.o gettimeofday.o datapage.o cacheflush.o note.o getcpu.o
 
 # Build rules
 

arch/powerpc/kernel/vdso32/getcpu.S

@@ -15,6 +15,7 @@
  * int __kernel_getcpu(unsigned *cpu, unsigned *node);
  *
  */
+#if defined(CONFIG_PPC64)
 V_FUNCTION_BEGIN(__kernel_getcpu)
   .cfi_startproc
 	mfspr	r5,SPRN_SPRG_VDSO_READ
@@ -31,3 +32,19 @@ V_FUNCTION_BEGIN(__kernel_getcpu)
 	blr
   .cfi_endproc
 V_FUNCTION_END(__kernel_getcpu)
+#elif !defined(CONFIG_SMP)
+V_FUNCTION_BEGIN(__kernel_getcpu)
+  .cfi_startproc
+	cmpwi	cr0, r3, 0
+	cmpwi	cr1, r4, 0
+	li	r5, 0
+	beq	cr0, 1f
+	stw	r5, 0(r3)
+1:	li	r3, 0			/* always success */
+	crclr	cr0*4+so
+	beqlr	cr1
+	stw	r5, 0(r4)
+	blr
+  .cfi_endproc
+V_FUNCTION_END(__kernel_getcpu)
+#endif

arch/powerpc/kernel/vdso32/vdso32.lds.S

@@ -155,7 +155,7 @@ VERSION
 		__kernel_sync_dicache_p5;
 		__kernel_sigtramp32;
 		__kernel_sigtramp_rt32;
-#ifdef CONFIG_PPC64
+#if defined(CONFIG_PPC64) || !defined(CONFIG_SMP)
 		__kernel_getcpu;
 #endif