Re: [Xen-devel] [PATCH RFC] x86: Add hack to disable "Fake HT" mode

[Andreas Kinzler <hfp@xxxxxxxxx>]

On 15.11.2019 11:57, 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 (among other things) actually reporting hyperthreading as
> available, but giving vcpus every other APICID.  The resulting cpu
> featureset is invalid, but most operating systems on most hardware
> managed to cope with it.
>
> Unfortunately, Windows running on modern AMD hardware -- including
> Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus -- gets
> confused by the resulting contradictory feature bits and crashes
> during installation.  (Linux guests have so far continued to cope.)
I do not understand a central point: No matter why and/or how a fake 
topology is presented by Xen, why did the older generation Ryzen 2xxx 
work and Ryzen 3xxx doesn't? What is the change in AMD(!) not Xen that 
causes the one to work and the other to fail?
Regards Andreas

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