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

[Sean Christopherson]

On Tue, Nov 05, 2019 at 11:37:50AM -0800, Sean Christopherson 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.
> 
> I'd stay away from "trustworthy", especially if this is controlled by
> userspace.  Whether or not the hint is trustworthy is purely up to the
> guest.  Right now it doesn't really matter, but that will change as we
> start moving pieces of the host out of the guest's TCB.
> 
> It may make sense to split the two (or even three?) cases, e.g.
> KVM_FEATURE_NO_SMT and KVM_FEATURE_ACCURATE_TOPOLOGY.  KVM can easily
> enforce NO_SMT _today_, i.e. allow it to be set if and only if SMT is
> truly disabled.  Verifying that the topology exposed to the guest is legit
> is a completely different beast.

Scratch the ACCURATE_TOPOLOGY idea, I doubt there's a real use case for
setting ACCURATE_TOPOLOGY and not KVM_HINTS_REALTIME.  A feature flag to
state that SMT is disabled seems simple and useful.