arm64: uaccess: permit __smp_store_release() to use zero register

Currently the asm constraints for __smp_store_release() require that the
value is placed in a "real" GPR (i.e. one other than [XW]ZR or SP).
This means that for cases such as:

    __smp_store_release(ptr, 0)

... the compiler has to move '0' into "real" GPR, e.g.

    mov     xN, #0
    stlr    xN, [<addr>]

This is unfortunate, as using the zero register would require fewer
instructions and save a "real" GPR for other usage, allowing the
compiler to generate:

    stlr    xzr, [<addr>]

Modify the asm constaints for __smp_store_release() to permit the use of
the zero register for the value.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230314153700.787701-3-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 3dd8982..cf29874 100644 (file)
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -131,25 +131,25 @@ do {                                                                      \
        case 1:                                                         \
                asm volatile ("stlrb %w1, %0"                           \
                                : "=Q" (*__p)                           \
-                               : "r" (*(__u8 *)__u.__c)                \
+                               : "rZ" (*(__u8 *)__u.__c)               \
                                : "memory");                            \
                break;                                                  \
        case 2:                                                         \
                asm volatile ("stlrh %w1, %0"                           \
                                : "=Q" (*__p)                           \
-                               : "r" (*(__u16 *)__u.__c)               \
+                               : "rZ" (*(__u16 *)__u.__c)              \
                                : "memory");                            \
                break;                                                  \
        case 4:                                                         \
                asm volatile ("stlr %w1, %0"                            \
                                : "=Q" (*__p)                           \
-                               : "r" (*(__u32 *)__u.__c)               \
+                               : "rZ" (*(__u32 *)__u.__c)              \
                                : "memory");                            \
                break;                                                  \
        case 8:                                                         \
-               asm volatile ("stlr %1, %0"                             \
+               asm volatile ("stlr %x1, %0"                            \
                                : "=Q" (*__p)                           \
-                               : "r" (*(__u64 *)__u.__c)               \
+                               : "rZ" (*(__u64 *)__u.__c)              \
                                : "memory");                            \
                break;                                                  \
        }                                                               \