Re: [PATCH v2 1/3] kvm: x86: Introduce hypercall x86 ops for handling hypercall not in cpl0

[Sean Christopherson]

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.