Re: [Xen-devel] [PATCH v5 06/10] arm: smccc: handle SMCs according to SMCCC

[Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>]

Hi Julien,

On 20.09.17 20:21, Julien Grall wrote:
>
>
> On 19/09/17 22:44, Volodymyr Babchuk wrote:
>> Hi Julien,
>
> Hi Volodymyr,
>
>>
>> On 13.09.17 14:11, Julien Grall wrote:
>>> Hi,
>>>
>>> On 08/31/2017 09:09 PM, Volodymyr Babchuk wrote:
>>
>>>> +static void fill_uuid(struct cpu_user_regs *regs, const 
xen_uuid_t *u)
>>>
>>> Actually why do you pass a pointer for u? This requires every caller
>>> to introduce temporary variable because the UUID is usually a define.
>> Hmm, another way probably is to pass a whole structure as a parameter.
>> Are you suggesting this approach? Something like
>> fill_uuid(regs, (xen_uuid_t)MY_UUID)?
>
> Something list that. But why do you need the cast? MY_UUID is supposed
> to be a xen_uuid_t. No?
It have no type. It is just an initializer list like {1,2,3,4,5,6}. If 
you remember that thread, there is a requirement to make public headers 
compatible with c89. So I can't define MY_UUID as (xen_uuid_t){1,2,3}.
Instead it is defined as a plain initializer list.

>>
>>> With your current solution each caller as to do:
>>>
>>> xen_uuid_t foo = MY_UUID;
>>>
>>> fill_uuid(regs, &foo);
>>>
>>> return true;
>>>
>>> What I suggested in the previous version is to get fill_uuid return
>>> true. So you make each caller simpler.
>> Yes, but it will not be correct semantically. There will arise many
>> questions:
>> 1. Why helper function that only writes data returns bool?
>> 2. If it returns true, can it return false?
>> 3. Should we check its return value before passing it further?
>
>
> I really don't see how
>
> return fill_uuid(regs, MY_UUID);
>
> would be semantically incorrect or even raise all those questions. It is
> perfectly fine to always return true. We have place like that and it
> helps to streamline the code.This is a somewhat arguable topic.
Yes, your variant produces less lines of code.
But it is harder to read, actually. `return fill_uid(regs, MY_UUID)`
implies that there are some logic in `fill_uid()` and it can return 
different results, depending on its parameters. Which is not true and, 
thus, misleading.
Just try to look at this from stranger's point of view. Usually you 
don't declare function as a `bool` just to spare a line of code. If you 
see boolean function, you expect that there are some reason, some logic 
behind this.
Look, I have an idea how to resolve this.
fill_uid() will check that MY_UUID != {ffffffff, fffff, fffff, ffff, 
ffffffffffff}.
If UUID is invalid, it will print warning to console and return false.
If UUID is valid, it will fill registers and return true.
Now it will be semantically correct to define it as `bool fill_uuid()`

This can work for `fill_uuid()`. But you also expressed the same idea 
regarding code that return version. I can create helper function that 
fills registers with version info. But there I don't see any excuse to 
declare that helper as boolean.
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