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.
