powerpc/time: Fix mftb()/get_tb() for use with the compat VDSO

When we're building the compat VDSO we are building 32-bit code but in
the context of a 64-bit kernel configuration.

To make this work we need to be careful in some places when using
ifdefs to differentiate between CONFIG_PPC64 and __powerpc64__.

CONFIG_PPC64 indicates the kernel we're building is 64-bit, but it
doesn't tell us that we're currently building 64-bit code - we could
be building 32-bit code for the compat VDSO.

On the other hand __powerpc64__ tells us that we are currently
building 64-bit code (and therefore we must also be building a 64-bit
kernel).

In the case of get_tb() we want to use the 32-bit code sequence
regardless of whether the kernel we're building for is 64-bit or
32-bit, what matters is the word size of the current object. So we
need to check __powerpc64__ to decide if we use mftb() or the
mftbu()/mftb() sequence.

For mftb() the logic for CPU_FTR_CELL_TB_BUG only makes sense if we're
building 64-bit code, so guard that with a __powerpc64__ check.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201126131006.2431205-4-mpe@ellerman.id.au

arch/powerpc/include/asm/vdso/timebase.h

@@ -8,7 +8,11 @@
 
 #include <asm/reg.h>
 
-#if defined(CONFIG_PPC_CELL) || defined(CONFIG_E500)
+/*
+ * We use __powerpc64__ here because we want the compat VDSO to use the 32-bit
+ * version below in the else case of the ifdef.
+ */
+#if defined(__powerpc64__) && (defined(CONFIG_PPC_CELL) || defined(CONFIG_E500))
 #define mftb()		({unsigned long rval;				\
 			asm volatile(					\
 				"90:	mfspr %0, %2;\n"		\
@@ -49,7 +53,11 @@ static inline u64 get_tb(void)
 {
 	unsigned int tbhi, tblo, tbhi2;
 
-	if (IS_ENABLED(CONFIG_PPC64))
+	/*
+	 * We use __powerpc64__ here not CONFIG_PPC64 because we want the compat
+	 * VDSO to use the 32-bit compatible version in the while loop below.
+	 */
+	if (__is_defined(__powerpc64__))
 		return mftb();
 
 	do {