[PATCH 1/6] arm64: sysreg: allow write_sysreg to use XZR

[Robin Murphy]

On 08/09/16 13:55, Mark Rutland wrote:
> 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.
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Marc Zyngier <marc.zyngier at arm.com>
> Cc: Robin Murphy <robin.murphy at arm.com>
> Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
> Cc: Will Deacon <will.deacon at arm.com>
> ---
>  arch/arm64/include/asm/sysreg.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index cc06794..39fed2e 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.
> + */

Personally I find that comment more confusing than useful - "Z" is
documented for AArch64 as "Integer constant zero", which to me doesn't
necessarily imply an immediate; it's just not necessarily immediately
intuitive that that can be an immediate constant or a 'register
constant' depending on exactly how it is referenced.

Either way, though:

Reviewed-by: Robin Murphy <robin.murphy at arm.com>

>  #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
>