Re: [PATCH 1/4] KVM: nVMX: support restore of VMX capability MSRs

[Paolo Bonzini <pbonzini@redhat.com>]

> On Mon, Nov 28, 2016 at 2:48 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > On 28/11/2016 22:11, David Matlack wrote:
> >> > PINBASED_CTLS, PROCBASED_CTLS, EXIT_CTLS and ENTRY_CTLS can be derived
> >> > from their "true" counterparts, so I think it's better to remove the
> >> > "non-true" ones from struct nested_vmx (and/or add the "true" ones when
> >> > missing) and make them entirely computed.  But it can be done on top.
> >>
> >> Good point. And that would mean userspace does not need to restore the
> >> non-true MSRs, right?
> >
> > Yes, sorry for being a bit too concise. :)
> 
> I'll include this cleanup in the next version of the patchset since it
> affects which MSRs userspace will restore. It looks like a pretty
> simple patch.

Don't bother removing the "non-true" registers from nested_vmx; you only
need to adjust the userspace API.

> >
> >> KVM does not emulate MSR_IA32_VMX_BASIC[55]=0,
> >> and will probably never want to.
> >
> > That's a separate question, MSR_IA32_VMX_BASIC[55]=0 basically means
> > that the "true" capabilities are the same as the "default" capabilities.
> >  If userspace wanted to set it that way, KVM right now would not hide
> > the "true" capability MSR, but on the other hand the nested hypervisor
> > should not even notice the difference.
> 
> KVM would also need to use the non-true MSR in place of the true MSRs
> when checking VMCS12 during VM-entry.

It's not necessary, userspace would set the relevant bits to 1 in the true
MSRs, for both the low and high parts.  If it doesn't, it's garbage in
garbage out.

Paolo