On Mon, Nov 25, 2019 at 12:39:23PM +0000, George Dunlap wrote:
> Changeset ca2eee92df44 ("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 <net...@crc.id.au>
> Reported-by: Andreas Kinzler <h...@posteo.de>
> Signed-off-by: George Dunlap <george.dun...@citrix.com>
> ---
> CC: Ian Jackson <ian.jack...@citrix.com>
> CC: Wei Liu <w...@xen.org>
> CC: Andrew Cooper <andrew.coop...@citrix.com>
> CC: Jan Beulich <jbeul...@suse.com>
> CC: Juergen Gross <jgr...@suse.com>
> ---
> tools/libxc/xc_cpuid_x86.c | 7 ++++++-
I will defer this to x86 maintainers.
Seeing that you already have an Ack from Jan, feel free to add mine if
necessary.
Wei.
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