powerpc: Add ZEROIZE_GPRS macros for register clears

Provide register zeroing macros, following the same convention as
existing register stack save/restore macros, to be used in later
change to concisely zero a sequence of consecutive gprs.

The resulting macros are called ZEROIZE_GPRS and ZEROIZE_NVGPRS, keeping
with the naming of the accompanying restore and save macros, and usage
of zeroize to describe this operation elsewhere in the kernel.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220921065605.1051927-4-rmclure@linux.ibm.com

arch/powerpc/include/asm/ppc_asm.h

@@ -33,6 +33,20 @@
 	.endr
 .endm
 
+/*
+ * This expands to a sequence of register clears for regs start to end
+ * inclusive, of the form:
+ *
+ *   li rN, 0
+ */
+.macro ZEROIZE_REGS start, end
+	.Lreg=\start
+	.rept (\end - \start + 1)
+	li	.Lreg, 0
+	.Lreg=.Lreg+1
+	.endr
+.endm
+
 /*
  * Macros for storing registers into and loading registers from
  * exception frames.
@@ -49,6 +63,14 @@
 #define REST_NVGPRS(base)		REST_GPRS(13, 31, base)
 #endif
 
+#define	ZEROIZE_GPRS(start, end)	ZEROIZE_REGS start, end
+#ifdef __powerpc64__
+#define	ZEROIZE_NVGPRS()		ZEROIZE_GPRS(14, 31)
+#else
+#define	ZEROIZE_NVGPRS()		ZEROIZE_GPRS(13, 31)
+#endif
+#define	ZEROIZE_GPR(n)			ZEROIZE_GPRS(n, n)
+
 #define SAVE_GPR(n, base)		SAVE_GPRS(n, n, base)
 #define REST_GPR(n, base)		REST_GPRS(n, n, base)