powerpc/uaccess: Use YZ asm constraint for ld

The 'ld' instruction requires a 4-byte aligned displacement because it
is a DS-form instruction. But the "m" asm constraint doesn't enforce
that.

Add a special case of __get_user_asm2_goto() so that the "YZ" constraint
can be used for "ld".

The "Z" constraint is documented in the GCC manual PowerPC machine
constraints, and specifies a "memory operand accessed with indexed or
indirect addressing". "Y" is not documented in the manual but specifies
a "memory operand for a DS-form instruction". Using both allows the
compiler to generate a DS-form "ld" or X-form "ldx" as appropriate.

The change has to be conditional on CONFIG_PPC_KERNEL_PREFIXED because
the "Y" constraint does not guarantee 4-byte alignment when prefixed
instructions are enabled.

No build errors have been reported due to this, but the possibility is
there depending on compiler code generation decisions.

Fixes: c20beffeec ("powerpc/uaccess: Use flexible addressing with __put_user()/__get_user()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240529123029.146953-2-mpe@ellerman.id.au

arch/powerpc/include/asm/uaccess.h

@@ -181,8 +181,19 @@ do {								\
 #endif
 
 #ifdef __powerpc64__
+#ifdef CONFIG_PPC_KERNEL_PREFIXED
 #define __get_user_asm2_goto(x, addr, label)			\
 	__get_user_asm_goto(x, addr, label, "ld")
+#else
+#define __get_user_asm2_goto(x, addr, label)			\
+	asm_goto_output(					\
+		"1:	ld%U1%X1 %0, %1	# get_user\n"		\
+		EX_TABLE(1b, %l2)				\
+		: "=r" (x)					\
+		: DS_FORM_CONSTRAINT (*addr)			\
+		:						\
+		: label)
+#endif // CONFIG_PPC_KERNEL_PREFIXED
 #else /* __powerpc64__ */
 #define __get_user_asm2_goto(x, addr, label)			\
 	asm_goto_output(					\