[PATCH v2 05/11] arm64: sve: Provide a conditional update accessor for ZCR_ELx

[Marc Zyngier]

On 2021-03-19 16:42, Mark Brown wrote:
> On Thu, Mar 18, 2021 at 12:25:26PM +0000, Marc Zyngier wrote:
> 
>> A common pattern is to conditionally update ZCR_ELx in order
>> to avoid the "self-synchronizing" effect that writing to this
>> register has.
>> 
>> Let's provide an accessor that does exactly this.
> 
> Reviewed-by: Mark Brown <broonie at kernel.org>
> 
>> +#define sve_cond_update_zcr_vq(val, reg)		\
>> +	do {						\
>> +		u64 __zcr = read_sysreg_s((reg));	\
>> +		u64 __new = __zcr & ~ZCR_ELx_LEN_MASK;	\
>> +		__new |= (val) & ZCR_ELx_LEN_MASK;	\
>> +		if (__zcr != __new)			\
>> +			write_sysreg_s(__new, (reg));	\
>> +	} while (0)
>> +
> 
> Do compilers actually do much better with this than with a static
> inline like the other functions in this header?  Seems like something
> they should be figuring out.

It's not about performance or anything of the sort: in most cases
where we end-up using this, it is on the back of an exception.
So performance is the least of our worries.

However, the "reg" parameter to read/write_sysreg_s() cannot
be a variable, because it is directly fed to the assembler.
If you want to use functions, you need to specialise them per
register. At this point, I'm pretty happy with a #define.

         M.
-- 
Jazz is not dead. It just smells funny...