Re: [RFC 14/33] KVM: x86: Add VTL to the MMU role

[Nicolas Saenz Julienne]

On Wed Nov 8, 2023 at 5:26 PM UTC, Sean Christopherson wrote:
> On Wed, Nov 08, 2023, Nicolas Saenz Julienne wrote:
> > With the upcoming introduction of per-VTL memory protections, make MMU
> > roles VTL aware. This will avoid sharing PTEs between vCPUs that belong
> > to different VTLs, and that have distinct memory access restrictions.
> >
> > Four bits are allocated to store the VTL number in the MMU role, since
> > the TLFS states there is a maximum of 16 levels.
>
> How many does KVM actually allow/support?  Multiplying the number of possible
> roots by 16x is a *major* change.

AFAIK in practice only VTL0/1 are used. Don't know if Microsoft will
come up with more in the future. We could introduce a CAP that expses
the number of supported VTLs to user-space, and leave it as a compile
option.