Re: [PATCH for-4.15] vtd: make sure QI/IR are disabled before initialisation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.03.2021 15:52, Igor Druzhinin wrote:
> On 08/03/2021 08:18, Jan Beulich wrote:
>> On 08.03.2021 08:00, Igor Druzhinin wrote:
>>> BIOS might pass control to Xen leaving QI and/or IR in enabled and/or
>>> partially configured state. In case of x2APIC code path where EIM is
>>> enabled early in boot - those are correctly disabled by Xen before any
>>> attempt to configure. But for xAPIC that step is missing which was
>>> proven to cause QI initialization failures on some ICX based platforms
>>> where QI is left pre-enabled and partially configured by BIOS.
>>
>> And those systems then tell us to avoid use of x2APIC? I would have
>> expected that on modern systems we wouldn't see such quirky firmware
>> behavior anymore. Anyway, half a sentence to this effect might help
>> here, as without such firmware behavior the only way to run into
>> this ought to be use of "no-x2apic" on the command line. Which in
>> turn might require justification (and potentially a fix elsewhere in
>> the code to make use of that option unnecessary).
>>
>>> Unify the behaviour between x2APIC and xAPIC code paths keeping that in
>>> line with what Linux does.
>>>
>>> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> with some editing of the description. If no other need for a v2
>> arises, I suppose whatever you come up with could be folded in
>> while committing.
> 
> How about:
> 
> "... But for xAPIC that step is missing which was proven to cause QI 
> initialization failures on some ICX based platforms where QI is left 
> pre-enabled and partially configured by BIOS. That problem becomes hard 
> to avoid since those platforms are shipped with x2APIC opt out being 
> advertised by default at the same time by firmware.
> ..."

SGTM.

Thanks, Jan