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

From: Yu Zhang
Date: Thu Sep 09 2021 - 21:54:02 EST

On Thu, Sep 09, 2021 at 05:09:11PM +0000, Sean Christopherson wrote:
> 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).

Oh, I see. It's to make the virtualized world more real. But like you said,
it's not KVM's target. And doing that could cause more problems - a PV guest
expects the VMCALL to succeed, regardless it has VMX capability or its VMX is
on or not.

Thanks for the explaination.