Changeset ca2eee92df ("x86, hvm: Expose host core/HT topology to HVM
guests") attempted to "fake up" a topology which would induce guest
operating systems to not treat vcpus as sibling hyperthreads. This
involved actually reporting hyperthreading as available, but giving
vcpus every other ApicId; which in turn led to doubling the ApicIds
per core by bumping the ApicIdCoreSize by one. In particular, Ryzen
3xxx series processors, and reportedly EPYC "Rome" cpus -- have an
ApicIdCoreSize of 7; the "fake" topology increases this to 8.
Unfortunately, Windows running on modern AMD hardware -- including
Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus --
doesn't seem to cope with this value being higher than 7. (Linux
guests have so far continued to cope.)
A "proper" fix is complicated and it's too late to fix it either for
4.13, or to backport to supported branches. As a short-term fix,
limit this value to 7.
This does mean that a Linux guest, booted on such a system without
this change, and then migrating to a system with this change, with
more than 64 vcpus, would see an apparent topology change. This is a
low enough risk in practice that enabling this limit unilaterally, to
allow other guests to boot without manual intervention, is worth it.
Reported-by: Steven Haigh <netwiz@crc.id.au>
Reported-by: Andreas Kinzler <hfp@posteo.de>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
George Dunlap
8c79c129a6