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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Also Cc-ing Ian for a release ack.

Jan