Re: [PATCH 11/15] KVM: VMX: Disable loading of TSX_CTRL MSR the more conventional way

[Paolo Bonzini]

On 05/05/21 17:36, Sean Christopherson wrote:
> On Wed, May 05, 2021, Paolo Bonzini wrote:
> > On 04/05/21 19:17, Sean Christopherson wrote:
> > > Tag TSX_CTRL as not needing to be loaded when RTM isn't supported in the
> > > host.  Crushing the write mask to '0' has the same effect, but requires
> > > more mental gymnastics to understand.
> >
> > This doesn't explain _why_ this is now possible.  What about:
> >
> > Now that user return MSRs is always present in the list, we don't have
>
> User return MSRs aren't always present in the list; this series doesn't change
> that behavior at all.
>
> > the problem that the TSX_CTRL MSR needs a slot vmx->guest_uret_msrs even
> > if RTM is not supported in the host (and therefore there is nothing to
> > enable).  Thus we can simply tag TSX_CTRL as not needing to be loaded
> > instead of crushing the write mask to '0'.
>
> Unless I'm missing something, it would have been possible to give TSX_CTRL a
> slot but not load it even before this refactoring, we just missed that approach
> when handling the TSX_CTRL without HLE/RTM case.  Several other MSRs rely on
> this behavior, notably the SYSCALL MSRs, which are present in the list so that
> the guest can read/write the MSRs, but are loaded into hardware iff the guest
> has enabled SYSCALL.

You're right, it used to be done with vmx->nr_active_uret_msr.

Paolo

> All that said, I certainly have no objection to writing a longer changelog.