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. >
