[BUG] x2apic broken with current AMD hardware

[Elliott Mitchell <ehem+xen@xxxxxxx>]

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


-- 
(\___(\___(\______          --=> 8-) EHM <=--          ______/)___/)___/)
 \BS (    |         ehem+sigmsg@xxxxxxx  PGP 87145445         |    )   /
  \_CS\   |  _____  -O #include <stddisclaimer.h> O-   _____  |   /  _/
8A19\___\_|_/58D2 7E3D DDF4 7BA6 <-PGP-> 41D1 B375 37D0 8714\_|_/___/5445