Re: [BUG] x2apic broken with current AMD hardware

[Elliott Mitchell <ehem+xen@xxxxxxx>]

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)


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