Re: [BUG] x2apic broken with current AMD hardware

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.03.2023 01:09, Elliott Mitchell wrote:
> On Thu, Mar 09, 2023 at 10:03:23AM +0100, Jan Beulich wrote:
>>
>> In any event you will want to collect a serial log at maximum verbosity.
>> It would also be of interest to know whether turning off the IOMMU avoids
>> the issue as well (on the assumption that your system has less than 255
>> CPUs).
> 
> I think I might have figured out the situation in a different fashion.
> 
> I was taking a look at the BIOS manual for this motherboard and noticed
> a mention of a "Local APIC Mode" setting.  Four values are listed
> "Compatibility", "xAPIC", "x2APIC", and "Auto".
> 
> That is the sort of setting I likely left at "Auto" and that may well
> result in x2 functionality being disabled.  Perhaps the x2APIC
> functionality on AMD is detecting whether the hardware is present, and
> failing to test whether it has been enabled?  (could be useful to output
> a message suggesting enabling the hardware feature)

Can we please move to a little more technical terms here? What is "present"
and "enabled" in your view? I don't suppose you mean the CPUID bit (which
we check) and the x2APIC-mode-enable one (which we drive as needed). It's
also left unclear what the four modes of BIOS operation evaluate to. Even
if we knew that, overriding e.g. "Compatibility" (which likely means some
form of "disabled" / "hidden") isn't normally an appropriate thing to do.
In "Auto" mode Xen likely should work - the only way I could interpret the
the other modes are "xAPIC" meaning no x2APIC ACPI tables entries (and
presumably the CPUID bit also masked), "x2APIC" meaning x2APIC mode pre-
enabled by firmware, and "Auto" leaving it to the OS to select. Yet that's
speculation on my part ...

Jan