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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan