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



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.
..."

Igor



 


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