x86/asm: Add _ASM_RIP() macro for x86-64 (%rip) suffix

Add a macro _ASM_RIP() to add a (%rip) suffix on 64 bits only. This is
useful for immediate memory references where one doesn't want gcc
to possibly use a register indirection as it may in the case of an "m"
constraint.

Signed-off-by: H. Peter Anvin (Intel) <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/include/asm/asm.h

···
 # define __ASM_FORM(x, ...)		x,## __VA_ARGS__
 # define __ASM_FORM_RAW(x, ...)		x,## __VA_ARGS__
 # define __ASM_FORM_COMMA(x, ...)	x,## __VA_ARGS__,
+# define __ASM_REGPFX			%
 #else
 #include <linux/stringify.h>
 # define __ASM_FORM(x, ...)		" " __stringify(x,##__VA_ARGS__) " "
 # define __ASM_FORM_RAW(x, ...)		    __stringify(x,##__VA_ARGS__)
 # define __ASM_FORM_COMMA(x, ...)	" " __stringify(x,##__VA_ARGS__) ","
+# define __ASM_REGPFX			%%
 #endif
 
 #define _ASM_BYTES(x, ...)	__ASM_FORM(.byte x,##__VA_ARGS__ ;)
···
 #define _ASM_BP		__ASM_REG(bp)
 #define _ASM_SI		__ASM_REG(si)
 #define _ASM_DI		__ASM_REG(di)
+
+/* Adds a (%rip) suffix on 64 bits only; for immediate memory references */
+#define _ASM_RIP(x)	__ASM_SEL_RAW(x, x (__ASM_REGPFX rip))
 
 #ifndef __x86_64__
 /* 32 bit */