Re: [PATCH v4 0/2] kvm/cpuid: set proper GuestPhysBits in CPUID.0x80000008

[Sean Christopherson <seanjc@google.com>]

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.