Re: [PATCH v4 1/8] KVM: nVMX: Keep KVM updates to BNDCFGS ctrl bits across MSR write

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 06, 2022, Oliver Upton wrote:
> Hey Sean,
> 
> On Wed, Apr 6, 2022 at 5:26 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Fri, Mar 04, 2022, Paolo Bonzini wrote:
> > > On 3/4/22 00:44, Sean Christopherson wrote:
> > > >
> > > > diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
> > > > index c92cea0b8ccc..46dd1967ec08 100644
> > > > --- a/arch/x86/kvm/vmx/nested.h
> > > > +++ b/arch/x86/kvm/vmx/nested.h
> > > > @@ -285,8 +285,8 @@ static inline bool nested_cr4_valid(struct kvm_vcpu *vcpu, unsigned long val)
> > > >  }
> > > >
> > > >  /* No difference in the restrictions on guest and host CR4 in VMX operation. */
> > > > -#define nested_guest_cr4_valid nested_cr4_valid
> > > > -#define nested_host_cr4_valid  nested_cr4_valid
> > > > +#define nested_guest_cr4_valid kvm_is_valid_cr4
> > > > +#define nested_host_cr4_valid  kvm_is_valid_cr4
> > >
> > > This doesn't allow the theoretically possible case of L0 setting some
> > > CR4-fixed-0 bits for L1.  I'll send another one.
> >
> > Are you still planning on sending a proper patch for this?
> >
> > And more importantly, have we shifted your view on this patch/series?
> 
> Sorry, I should've followed up. If nobody else complains, let's just
> leave everything as-is and avoid repeating the mistakes of the patches
> to blame (hey, I authored one of those!)

The problem is that if we leave things as is, someone will inevitably think it's
the right thing to do and will repeat those mistakes.  I don't see why we wouldn't
add the quirk, broken userspace gets to keep its broken behavior unless it opts
into to disabling the quirk.