Re: [PATCH] xen/arm: minor improvement in smp_send_call_function_mask()

[Julien Grall <julien.grall@linaro.org>]

On 20/08/14 01:14, Anup Patel wrote:
>> Is it necessary? What happen if Xen tries to send an SGI with an empty mask?
>
> If Xen tries to send SGI with empty mask then target_mask will be empty
> hence cpumask_weight(&target_mask) will return 0

The GIC documentation says:
"If this field is 0x00 when TargetListFilter is 0b00 , the Distributor 
does not forward the interrupt to any
CPU interface."

TargetListFilter == 0b00 => Forward interrupt to a specific list of CPUs.

>>
>> AFAIU, the function cpumask_weight is complex so if we can avoid it, it
>> would be better.
>
> Can you explain more about how cpumask_weight is complex ??

cpumask_weight contains a loop and multiple addition. I doubt the case 
where the mask only contains the current cpu happens often.

As the GIC won't forward the interrupt if the list of CPUs is empty, I 
don't think it's worth to add this check.

> Other alternative is to use "cpumask_first(&target_mask) != NR_CPUS".

The best alternative would be cpumask_empty.

Regards,

-- 
Julien Grall