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

From: Juergen Gross
Date: Wed Jun 12 2019 - 11:03:54 EST


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