On Fri, Sep 10, 2021, Yu Zhang wrote: > On Thu, Sep 09, 2021 at 07:55:23PM +0800, Hou Wenlong wrote: > > Per Intel's SDM, use vmcall instruction in non VMX operation for cpl3 > > it should trigger a #UD. And in VMX root operation, it should > > Are you sure? IIRC, vmcall will always cause VM exit as long as CPU > is in non-root mode(regardless the CPL). Correct, VMCALL unconditionally causes VM-Exit in non-root mode, but Hou is referring to the first fault condition of "non VMX operation". The intent of the patch is to emulate hardware behavior for CPL>0: if L1 is not in VMX operation, a.k.a. not post-VMXON, then #UD, else #GP (because VMCALL #GPs at CPL>0 in VMX root). On one hand, I agree with Hou's logic; injecting #UD/#GP is architecturally correct if KVM is emulating a bare metal environment for the guest. On the other hand, that contradicts with KVM _not_ injecting #UD for guest CPL0, i.e. KVM is clearly not emulating a bare metal environment. In the end, this would represent an ABI change for guest CPL>0. While it's highly unlikely that such a change would cause problems, maintaining the current behavior is the safe option unless there's strong motivation for changing the guest ABI. And injecting #UD/#GP would also mean KVM would again have to change its ABI if there is a future hypercall KVM wants to allow at CPL>0. Again, that's unlikely, but again I don't see sufficient justification.
