Re: [PATCH 2/2] KVM: VMX: Add knob to allow rejecting kvm_intel on inconsistent VMCS config

[Sean Christopherson <seanjc@google.com>]

On Wed, May 25, 2022, Jim Mattson wrote:
> On Wed, May 25, 2022 at 2:04 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > Add an off-by-default module param, reject_inconsistent_vmcs_config, to
> > allow rejecting the load of kvm_intel if an inconsistent VMCS config is
> > detected.  Continuing on with an inconsistent, degraded config is
> > undesirable when the CPU is expected to support a given set of features,
> > e.g. can result in a misconfigured VM if userspace doesn't cross-check
> > KVM_GET_SUPPORTED_CPUID, and/or can result in poor performance due to
> > lack of fast MSR switching.
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> There are several inconsistent VMCS configs that are not rejected here
> (e.g. "enable XSAVES/XRSTORS" on a CPU that doesn't support XSAVES).
> Do you plan to include more checks in the future, or should this be,
> "reject_some_inconsistent_vmcs_configs"? :-)

I have no plan, it was purely a reaction to continuing on with a known bad entry/exit
pair handling being awful.  I hesitated to even apply it to the EPT/VPID stuff, but
again it seemed silly to detect an inconsistency and do nothing about it.

I'm not opposed to adding more checks, though there is definitely a point of
diminishing returns.  I'm just picking the really low hanging fruit :-)