Re: [PATCH 0/3] xen: remove some checks for always present Xen features

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.04.2021 17:28, Juergen Gross wrote:
> On 22.04.21 17:23, Jan Beulich wrote:
>> On 22.04.2021 17:17, Juergen Gross wrote:
>>> On 22.04.21 17:16, Jan Beulich wrote:
>>>> On 22.04.2021 17:10, Juergen Gross wrote:
>>>>> Some features of Xen can be assumed to be always present, so add a
>>>>> central check to verify this being true and remove the other checks.
>>>>>
>>>>> Juergen Gross (3):
>>>>>     xen: check required Xen features
>>>>>     xen: assume XENFEAT_mmu_pt_update_preserve_ad being set for pv guests
>>>>>     xen: assume XENFEAT_gnttab_map_avail_bits being set for pv guests
>>>>
>>>> I wonder whether it's a good idea to infer feature presence from
>>>> version numbers. If (at some point in the past) you had inferred
>>>> gnttab v2 being available by version, this would have been broken
>>>> by its availability becoming controllable by a command line option
>>>> in Xen.
>>>
>>> I'm testing the feature to be really present when booting and issue a
>>> message if it is not there.
>>
>> And how does this help if the feature really isn't there yet other code
>> assumes it is?
> 
> Did you look at the features I'm testing?

I did, yes.

> Those are really just low
> level additions I can't imagine will ever be removed again.

I don't expect them to be removed. But I don't think the people having
contributed gnttab v2 expected any such for it, either.

Jan