Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is trustworthy

Peter Zijlstra
2019-11-06T08:32:54Z

On Tue, Nov 05, 2019 at 03:25:28PM -0800, Sean Christopherson wrote:
> On Tue, Nov 05, 2019 at 09:02:18PM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 05, 2019 at 05:17:37PM +0100, Vitaly Kuznetsov wrote:
> > > Virtualized guests may pick a different strategy to mitigate hardware
> > > vulnerabilities when it comes to hyper-threading: disable SMT completely,
> > > use core scheduling, or, for example, opt in for STIBP. Making the
> > > decision, however, requires an extra bit of information which is currently
> > > missing: does the topology the guest see match hardware or if it is 'fake'
> > > and two vCPUs which look like different cores from guest's perspective can
> > > actually be scheduled on the same physical core. Disabling SMT or doing
> > > core scheduling only makes sense when the topology is trustworthy.
> > >
> > > Add two feature bits to KVM: KVM_FEATURE_TRUSTWORTHY_SMT with the meaning
> > > that KVM_HINTS_TRUSTWORTHY_SMT bit answers the question if the exposed SMT
> > > topology is actually trustworthy. It would, of course, be possible to get
> > > away with a single bit (e.g. 'KVM_FEATURE_FAKE_SMT') and not lose backwards
> > > compatibility but the current approach looks more straightforward.
> >
> > The only way virt topology can make any sense what so ever is if the
> > vcpus are pinned to physical CPUs.
> >
> > And I was under the impression we already had a bit for that (isn't it
> > used to disable paravirt spinlocks and the like?). But I cannot seem to
> > find it in a hurry.
>
> Yep, KVM_HINTS_REALTIME does what you describe.

*sigh*, that's a pretty shit name for it :/