Re: [BUG] x2apic broken with current AMD hardware

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.03.2023 16:23, Elliott Mitchell wrote:
> Mostly SSIA.  As originally identified by "Neowutran", appears Xen's
> x2apic wrapper implementation fails with current generation AMD hardware
> (Ryzen 7xxx/Zen 4).  This can be worked around by passing "x2apic=false"
> on Xen's command-line, though I'm wondering about the performance impact.
> 
> There hasn't been much activity on xen-devel WRT x2apic, so a patch which
> fixed this may have flown under the radar.  Most testing has also been
> somewhat removed from HEAD.
> 
> Thanks to "Neowutran" for falling on this grenade and making it easier
> for the followers.  Pointer to first report:
> https://forum.qubes-os.org/t/ryzen-7000-serie/14538

I'm sorry, but when you point at this long a report, would you please be a
little more specific as to where the problem in question is actually
mentioned? Searching the page for "x2apic" didn't give any hits at all
until I first scrolled to the bottom of the (at present) 95 comments. And
then while there are five mentions, there's nothing I could spot that
would actually help understanding what is actually wrong. A statement like
"... is because the implementation of x2apic is incorrect" isn't helpful
on its own. And a later statement by another person puts under question
whether "x2apic=false" actually helps in all cases.

Please can we get a proper bug report here with suitable technical detail?
Or alternatively a patch to discuss?

Jan