Re: [PATCH v8 16/19] hvf: arm: Implement PSCI handling

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

+Markus

On 9/13/21 12:06 PM, Alexander Graf wrote:
> On 12.09.21 23:40, Richard Henderson wrote:
>> On 9/12/21 2:37 PM, Alexander Graf wrote:
>>>
>>> On 12.09.21 23:20, Richard Henderson wrote:
>>>> On 9/12/21 1:36 PM, Alexander Graf wrote:
>>>>>> I think the callsites would be clearer if you made the function
>>>>>> return true for "PSCI call handled", false for "not recognised,
>>>>>> give the guest an UNDEF". Code like
>>>>>>            if (hvf_handle_psci_call(cpu)) {
>>>>>>                stuff;
>>>>>>            }
>>>>>>
>>>>>> looks like the 'stuff' is for the "psci call handled" case,
>>>>>> which at the moment it isn't.
>>>>>
>>>>>
>>>>> This function merely follows standard C semantics along the lines
>>>>> of "0
>>>>> means success, !0 is error". Isn't that what you would usually expect?
>>>>
>>>> No, not really.  I expect stuff that returns error codes to return
>>>> negative integers on failure.  I expect stuff that returns a boolean
>>>> success/failure to return true on success.
>>>
>>>
>>> Fair, I'll change it to return -1 then. Thanks!
>>
>> Not quite the point I was making.  If the only two return values are
>> -1/0, then bool false/true is in fact more appropriate.
> 
> 
> If the whole code base adheres to it, maybe. The big problem with using
> true/false as return values for functions that don't make it very
> explicit (have an is in their name or use gerund for example). QEMU uses
> a lot of 0 == success internally.
> 
> If you now add bools to the a code base that already uses int returns a
> lot, you always need to look up what a function actually returns if the
> caller treats it as bool (if, ?, assert, etc), making code review and
> reasoning about code flows extremely hard. I've had one too many
> occasions where I called an innocently looking API and put the assert()
> the wrong way around for example.
> 
> I don't think we can solve this problem here, but IMHO the only sensible
> way to get to good readability would be to have functions that return
> success/failure return a libc defined struct that indicates the error.
> Then check for success explicitly on every caller site. Overloading bool
> or int for success/failure return is just bad :).

IIUC what we are trying to do lately is, if caller only expects
success/failure then return boolean, with error information in
Error* for eventual propagation. Only return libc errno if the
caller has to handle failures differently.