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

Peter Zijlstra
2019-11-05T20:02:32Z

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.

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.