arm64: sysreg: allow write_sysreg to use XZR

Currently write_sysreg has to allocate a temporary register to write
zero to a system register, which is unfortunate given that the MSR
instruction accepts XZR as an operand.

Allow XZR to be used when appropriate by fiddling with the assembly
constraints.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index cc06794b7346..39fed2e56e98 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -273,10 +273,14 @@ static inline void config_sctlr_el1(u32 clear, u32 set)
         __val;                                                  \
 })
 
+/*
+ * The "Z" constraint normally means a zero immediate, but when combined with
+ * the "%x0" template means XZR.
+ */
 #define write_sysreg(v, r) do {                                 \
         u64 __val = (u64)v;                                     \
-        asm volatile("msr " __stringify(r) ", %0"               \
-                     : : "r" (__val));                          \
+        asm volatile("msr " __stringify(r) ", %x0"              \
+                     : : "rZ" (__val));                         \
 } while (0)
 
 #endif