On Mon, Apr 15, 2024, Xiaoyao Li wrote: > On 4/12/2024 11:48 PM, Sean Christopherson wrote: > > On Fri, Apr 12, 2024, Xiaoyao Li wrote: > > If we go deep enough, it becomes a functional problem. It's not even _that_ > > ridiculous/contrived :-) > > > > L1 KVM is still aware that the real MAXPHYADDR=52, and so there are no immediate > > issues with reserved bits at that level. > > > > But L1 userspace will unintentionally configure L2 with CPUID.0x8000_0008.EAX[7:0]=48, > > and so L2 KVM will incorrectly think bits 51:48 are reserved. If both L0 and L1 > > are using TDP, neither L0 nor L1 will intercept #PF. And because L1 userspace > > was told MAXPHYADDR=48, it won't know that KVM needs to be configured with > > allow_smaller_maxphyaddr=true in order for the setup to function correctly. > > In this case, a) L1 userspace was told by L1 KVM that MAXPHYADDR = 48 via > KVM_GET_SUPPORTED_CPUID. But b) L1 userspace gets MAXPHYADDR = 52 by > executing CPUID itself. KVM can't assume userspace will do raw CPUID.
