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

Paolo Bonzini
2019-11-05T23:51:41Z

On 05/11/19 21:02, 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.

This is a subset of the requirements for "trustworthy" SMT. You can have:

- vCPUs pinned to two threads in the same core and exposed as multiple
cores in the guest

- vCPUs from different guests pinned to two threads in the same core

and that would be okay as far as KVM_HINTS_REALTIME is concerned, but
would still allow exploitation of side-channels, respectively within the
VM and between VMs.

Paolo

> 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.
>