selftests/powerpc: Fix pkey access right updates

The Power ISA mandates that all writes to the Authority
Mask Register (AMR) must always be preceded as well as
succeeded by a context synchronizing instruction.

This makes sure that the tests follow this requirement
when attempting to update a pkey's access rights.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200604125610.649668-2-sandipan@linux.ibm.com

tools/testing/selftests/powerpc/include/reg.h

@@ -57,6 +57,12 @@
 #define SPRN_PPR       896	/* Program Priority Register */
 #define SPRN_AMR       13	/* Authority Mask Register - problem state */
 
+#define set_amr(v)	asm volatile("isync;" \
+				     "mtspr " __stringify(SPRN_AMR) ",%0;" \
+				     "isync" : \
+				    : "r" ((unsigned long)(v)) \
+				    : "memory")
+
 /* TEXASR register bits */
 #define TEXASR_FC	0xFE00000000000000
 #define TEXASR_FP	0x0100000000000000

tools/testing/selftests/powerpc/ptrace/core-pkey.c

@@ -150,7 +150,7 @@ static int child(struct shared_info *info)
 	printf("%-30s AMR: %016lx pkey1: %d pkey2: %d pkey3: %d\n",
 	       user_write, info->amr, pkey1, pkey2, pkey3);
 
-	mtspr(SPRN_AMR, info->amr);
+	set_amr(info->amr);
 
 	/*
 	 * We won't use pkey3. This tests whether the kernel restores the UAMOR

tools/testing/selftests/powerpc/ptrace/ptrace-pkey.c

@@ -126,7 +126,7 @@ static int child(struct shared_info *info)
 	printf("%-30s AMR: %016lx pkey1: %d pkey2: %d pkey3: %d\n",
 	       user_write, info->amr1, pkey1, pkey2, pkey3);
 
-	mtspr(SPRN_AMR, info->amr1);
+	set_amr(info->amr1);
 
 	/* Wait for parent to read our AMR value and write a new one. */
 	ret = prod_parent(&info->child_sync);