Re: [ANNOUNCE] Xen 4.15 - release status, branching tomorrow

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.03.2021 17:53, Ian Jackson wrote:
> Jan Beulich writes ("Re: [ANNOUNCE] Xen 4.15 - release status, branching 
> tomorrow"):
>> On 23.03.2021 16:17, Ian Jackson wrote:
>>>   I think it may be time to reconcile ourselves to not fixing this,
>>>   and deciding on a suitable plan B.  Do we need to put something in
>>>   the release notes, or SUPPORT.md, or implement a mitigation of some
>>>   kind ?
>>
>> One option of course is, like was just done for 4.13.3, to revert.
>> Iirc Andrew had some thoughts towards making the new piece of code
>> conditional upon the original issue actually hitting.
> 
> I would be very happy to consider a revert it someone would give me
> references and explain to me the implications in words of one
> syllable.

Reference: e1de4c196a2e "x86/timer: Fix boot on Intel systems using
ITSSPRC static PIT clock gating"

Reverting would unbreak Xen on the Ryzen 1800X system where the
breakage was reported for, and likely a few others. It would at the
same time re-introduce Xen failing to boot on at least some Icelake
(and yet newer) systems.

>> Another
>> (somewhat similar) option might be to hide the new piece of code
>> behind a default-off command line option.
> 
> Likewise.

Well, not sure what to say here. Introducing a command line option
to allow making Icelake systems boot (by use of the option) while
keeping things working by default on older hardware is about as
simple as an explanation here can get, I guess.

Jan