Re: [RFC PATCH for 4.15 09/14] Provide cpu_opv system call

[Mathieu Desnoyers]

----- On Oct 13, 2017, at 10:50 PM, Andi Kleen andi@xxxxxxxxxxxxxx wrote:

>> +	pagefault_disable();
>> +	switch (len) {
>> +	case 1:
>> +		if (__get_user(tmp._u8, (uint8_t __user *)p))
>> +			goto end;
>> +		tmp._u8 += (uint8_t)count;
>> +		if (__put_user(tmp._u8, (uint8_t __user *)p))
>> +			goto end;
>> +		break;
> 
> It seems the code size could be dramatically shrunk by using a function
> pointer callback for the actual operation, and then avoiding all the
> copies. Since this is only a fallback this shouldn't be a problem
> (and modern branch predictors are fairly good in such situations anyways)
> 
> If you really want to keep the code it would be better if you used
> some macros -- i bet there's a typo in here somewhere in this
> forest.

Good point!

I'll add this in the next round. Meanwhile, here is the resulting
commit: https://github.com/compudj/linux-percpu-dev/commit/864b444f64f4c227ddc587d12631ff0d3440796c

Thanks,

Mathieu


> 
> -Andi

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com