Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 01, 2021, Paolo Bonzini wrote:
> On 01/02/21 17:38, Sean Christopherson wrote:
> > > > >      /*
> > > > >       * On TAA affected systems:
> > > > >       *      - nothing to do if TSX is disabled on the host.
> > > > >       *      - we emulate TSX_CTRL if present on the host.
> > > > >       *      This lets the guest use VERW to clear CPU buffers.
> > > > >       */
> > 
> > it says "nothing to do..." and then clears a
> > flag.  The other interpretation of "nothing to do... at runtime" is also wrong
> > as KVM emulates the MSR as a nop.
> > 
> > I guess I just find the whole comment more confusing than the code itself.
> 
> What about:
> 
> 
>         if (!boot_cpu_has(X86_FEATURE_RTM)) {
>                 /*
>                  * If RTM=0 because the kernel has disabled TSX, the host might
>                  * have TAA_NO or TSX_CTRL.  Clear TAA_NO (the guest sees RTM=0
>                  * and therefore knows that there cannot be TAA) but keep
>                  * TSX_CTRL: some buggy userspaces leave it set on tsx=on hosts,
>                  * and we want to allow migrating those guests to tsx=off hosts.
>                  */
>                 data &= ~ARCH_CAP_TAA_NO;
>         } else if (!boot_cpu_has_bug(X86_BUG_TAA)) {
>                 data |= ARCH_CAP_TAA_NO;
>         } else {
>                 /*
>                  * Nothing to do here; we emulate TSX_CTRL if present on the
>                  * host so the guest can choose between disabling TSX or
>                  * using VERW to clear CPU buffers.
>                  */
>         }

Awesome!  Thanks much!