On Tue, Nov 05, 2019 at 09:02:18PM +0100, 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. > > 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. Yep, KVM_HINTS_REALTIME does what you describe. > 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.
