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!
