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

Christophe de Dinechin
2019-11-08T15:35:24Z

> On 7 Nov 2019, at 16:02, Liran Alon <liran.alon@oracle.com> wrote:
>
>
>
>> On 7 Nov 2019, at 16:00, Christophe de Dinechin <christophe.de.dinechin@gmail.com> wrote:
>>
>
>>
>> I share that concern about the naming, although I do see some
>> value in exposing the cpu_smt_possible() result. I think it’s easier
>> to state that something does not work than to state something does
>> work.
>>
>> Also, with respect to mitigation, we may want to split the two cases
>> that Paolo outlined, i.e. have KVM_HINTS_REALTIME,
>> KVM_HINTS_CORES_CROSSTALK and
>> KVM_HINTS_CORES_LEAKING,
>> where CORES_CROSSTALKS indicates there may be some
>> cross-talk between what the guest thinks are isolated cores,
>> and CORES_LEAKING indicates that cores may leak data
>> to some other guest.
>>
>> The problem with my approach is that it is shouting “don’t trust me”
>> a bit too loudly.
>
> I don’t see a value in exposing CORES_LEAKING to guest. As guest have nothing to do with it.

The guest could display / expose the information to guest sysadmins
and admin tools (e.g. through /proc).

While the kernel cannot mitigate, a higher-level product could for example
have a policy about which workloads can be deployed on a system which
may leak data to other VMs.

Christophe