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

Sean Christopherson
2019-11-05T23:25:32Z

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.

> So I would much rather you have a bit that indicates the 1:1 vcpu/cpu
> mapping and if that is set accept the topology information and otherwise
> completely ignore it.