Re: [PATCH 1/2] common: map_vcpu_info() cosmetics

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.07.2020 18:17, Julien Grall wrote:
> On Thu, 16 Jul 2020, 17:01 Jan Beulich, <jbeulich@xxxxxxxx> wrote:
> 
>> On 16.07.2020 16:42, Roger Pau Monné wrote:
>>> On Thu, Jul 16, 2020 at 01:48:51PM +0200, Jan Beulich wrote:
>>>> On 16.07.2020 13:41, Roger Pau Monné wrote:
>>>>> On Wed, Jul 15, 2020 at 12:15:10PM +0200, Jan Beulich wrote:
>>>>>> Use ENXIO instead of EINVAL to cover the two cases of the address not
>>>>>> satisfying the requirements. This will make an issue here better stand
>>>>>> out at the call site.
>>>>>
>>>>> Not sure whether I would use EFAULT instead of ENXIO, as the
>>>>> description of it is 'bad address' which seems more inline with the
>>>>> error that we are trying to report.
>>>>
>>>> The address isn't bad in the sense of causing a fault, it's just
>>>> that we elect to not allow it. Hence I don't think EFAULT is
>>>> suitable. I'm open to replacement suggestions for ENXIO, though.
>>>
>>> Well, using an address that's not properly aligned to the requirements
>>> of an interface would cause a fault? (in this case it's a software
>>> interface, but the concept applies equally).
>>
>> Not necessarily, see x86'es behavior. Also even on strict arches
> 
> it is typically possible to cover for the misalignment by using
>> suitable instructions; it's still an implementation choice to not
>> do so.
> 
> 
> I am not sure about your argument here... Yes it might be possible, but at
> what cost?

The cost is what influences the decision whether to support it. Nevertheless
it remains an implementation decision rather than a hardware imposed
restriction, and hence I don't consider -EFAULT suitable here.

>>> Anyway, not something worth arguing about I think, so unless someone
>>> else disagrees I'm fine with using ENXIO.
>>
>> Good, thanks.
>>
> 
> -EFAULT can be described as "Bad address". I think it makes more sense than
> -ENXIO here even if it may not strictly result to a fault on some arch.

As said - I don't consider EFAULT applicable here; I also consider EINVAL
as too generic. I'll be happy to see replacement suggestions for my ENXIO
choice, but obviously I'm not overly happy to see options re-suggested
which I did already say I've ruled out.

Jan