Re: [Xen-devel] [PATCH v3 for-next 0/4] xen: Convert __page_to_mfn and _mfn_to_page to use typesafe MFN

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.11.17 at 13:16, <julien.grall@xxxxxxxxxx> wrote:

> 
> On 06/11/17 12:11, Jan Beulich wrote:
>>>>> On 06.11.17 at 12:47, <julien.grall@xxxxxxxxxx> wrote:
>>> Hi Jan,
>>>
>>> On 06/11/17 11:37, Jan Beulich wrote:
>>>>>>> On 01.11.17 at 15:03, <julien.grall@xxxxxxxxxx> wrote:
>>>>> Most of the users of page_to_mfn and mfn_to_page are either overriding
>>>>> the macros to make them work with mfn_t or use mfn_x/_mfn becaue the rest
>>>>> of the function use mfn_t.
>>>>>
>>>>> So I think it is time to make __page_to_mfn and __mfn_to_page using 
>>>>> typesafe
>>>>> MFN.
>>>>
>>>> I have to admit that I still find the overall goal confusing: Afaict
>>>> the double-underscore-prefixed versions exist only to allow
>>>> easily overriding the non-prefixed ones. Hence the first and
>>>> foremost goal ought to be to convert everyone to using the
>>>> non-prefixed versions. Files wanting to avoid the typed forms
>>>> could then continue to use / be switched to the prefixed ones.
>>>>
>>>> What you're doing here is producing a mess: The prefixed
>>>> versions should never have been touched in the first place.
>>>> And iirc this was discussed before, with the suggestion to use
>>>> overrides (for the non-prefixed versions) to limit overall patch
>>>> size.
>>>
>>> At the end of the discussion in the previous version, you were happy
>>> with the modification done here (see [1]).
>> 
>>  From the angle looked at it back then I indeed was, but I'm looking
>> at this from a slightly different angle now with the reply above.
>> 
>>> Overall, I think this is an improvement compare to what we have today.
>>> Because we enforce the use of MFN typesafe by default. The developer
>>> would have to override the helpers if he wants to to use the
>>> non-typesafe version.
>>>
>>> With your suggestion here, you would just keep the override around even
>>> when they are not necessary. They will have to be dropped at some point,
>>> so why not now?
>> 
>> Why would we keep the overrides in place once no longer needed?
>> All I'm asking for is that the double-underscore prefixed macros be
>> left alone, and the non-prefixed versions be converted to be
>> type-safe by default (with the option to override). That'll allow the
>> prefixed variants to go way altogether once all code was switched
>> to typesafe, no longer requiring any overrides.
> 
> If you left the double-underscore alone, then you can't convert headers 
> using them to typesafe. This is because they can't use the non-prefixed 
> version as they may be override.
> 
> So what you are suggesting here is will avoid converting those headers 
> until someone step up and finish to convert all the source to MFN 
> typesafe. Personally, I find quite silly to have to delay that...

Hmm, I see your point, but if we went the route you suggest,
what would be the steps to reach the ultimate result I've
described (the prefixed variants gone)? I seems to me that
this would require touching a lot of code a second time that is
being touched now, or is going to be touched as further
conversion happens.

Otoh, considering all the acks you've got already, if I'm the
only one thinking that switching the prefixed ones around is
the wrong thing, I perhaps shouldn't stand in the way of this
patch going in as is.

Jan


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