Re: [PATCH for-4.17] x86/hvm: Revert per-domain APIC acceleration support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.11.2022 19:14, Andrew Cooper wrote:
> On 15/11/2022 08:49, Jan Beulich wrote:
>> On 15.11.2022 01:35, Andrew Cooper wrote:
>>> I was really hoping to avoid this, but its now too late in the 4.17 freeze 
>>> and
>>> we still don't have working fixes.
>>>
>>> The in-Xen calculations for assistance capabilities are buggy.  For the
>>> avoidance of doubt, the original intention was to be able to control every
>>> aspect of a APIC acceleration so we could comprehensively test Xen's 
>>> support,
>>> as it has proved to be buggy time and time again.
>>>
>>> Even after a protracted discussion on what the new API ought to mean, 
>>> attempts
>>> to apply it to the existing logic have been unsuccessful, proving that the
>>> API/ABI is too complicated for most people to reason about.
>> Like Roger I'm still having trouble seeing what ABI you're talking
>> about here. Yes, there are internal handling issues, but that's hardly
>> "ABI". And as Roger indicated before, anything domctl/sysctl isn't
>> stable anyway.
> 
> It absolutely is stable when it it extends beyond domctl/sysctl into the
> libxl API, two different xl config files, and Xen command line (for PVH
> dom0).
> 
> domctl/sysctl are the very least of the problem.

Which still leaves open which "ABI" you've been referring to. As Roger
and I have said - it doesn't look as if we couldn't change the meaning
we internally assign to what libxl has started to expose (and what the
Xen command line option simply mirrors).

Jan