Re: [PATCH] x86/ucode: Refresh raw CPU policy after microcode load

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.05.2023 10:22, Roger Pau Monné wrote:
> On Thu, May 04, 2023 at 09:08:02AM +0100, Andrew Cooper wrote:
>> On 04/05/2023 8:08 am, Jan Beulich wrote:
>>> On 03.05.2023 20:58, Andrew Cooper wrote:
>>>> Loading microcode can cause new features to appear.
>>> Or disappear (LWP)? While I don't think we want to panic() in this
>>> case (we do on the S3 resume path when recheck_cpu_features() fails
>>> on the BSP), it would seem to me that we want to go a step further
>>> than you do and at least warn when a feature went amiss. Or is that
>>> too much of a scope-creeping request right here?
>>
>> You're correct that I ought to discuss the disappear case.  But like
>> livepatching, it's firmly in the realm of "the end user takes
>> responsibility for trying this in their test system before running it in
>> production".
>>
>> For LWP specifically, we ought to explicitly permit its disappearance in
>> recheck_cpu_features(), because this specific example is known to exist,
>> and known safe as Xen never used or virtualised LWP functionality. 
>> Crashing on S3
> 
> Printing disappearing features might be a nice bonus, but could be
> done from the tool that loads the microcode itself, no need to do such
> work in the hypervisor IMO.

Except that the system may not last long enough for the (or any) tool
to actually make it to producing such output, let alone any human
actually observing it (when that output isn't necessarily going to
some remote system).

Jan