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

[Juergen Gross]

On 12.06.19 12:19, Jan Beulich wrote:
> > > > On 12.06.19 at 12:12, <jgross@xxxxxxxx> 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.
>
> I don't agree with the second sentence: It is in principle fine for the
> hypervisor to expose HT (i.e. not disable it as bug mitigation), and
> leave it to the guest kernels to protect themselves. We're just not
> at the point yet where Xen offers sufficient / reliable data to guest
> kernels to do so, so _for the time being_ what you say is correct.

Okay, I'll add something like:

This is true as long Xen doesn't support core scheduling together with
exposing the (then) correct sibling information to the guest and
indicating that case via a sutable interface.


Juergen