Re: [PATCH] x86/xen: disable nosmt in Xen guests

[Juergen Gross]

On 12.06.19 13:48, Peter Zijlstra wrote:
> On Wed, Jun 12, 2019 at 12:12:28PM +0200, Juergen Gross wrote:
> > When running as a Xen guest selecting "nosmt" either via command line
> > or implicitly via default settings makes no sense, as the guest has no
> > clue about the real system topology it is running on. With Xen it is
> > the hypervisor's job to ensure the proper bug mitigations are active
> > regarding smt settings.
> >
> > So when running as a Xen guest set cpu_smt_control to "not supported"
> > in order to avoid disabling random vcpus.
>
> If it doesn't make sense; then the topology should not expose SMT
> threads and the knob will not have any effect.

Yes, that's the theory.

I agree completely, but this is ongoing work which will need some more
time. It probably would have been ready for some time now, but some
recent processor bugs required a shift in priorities what to do first.

In order to run a new kernel on existing Xen we need that patch to avoid
disabling random cpus.


Juergen