On Wed, Mar 17, 2021 at 3:19 PM Sean Christopherson <sea...@google.com> wrote: > > On Wed, Mar 17, 2021, Sean Christopherson wrote: > > On Wed, Mar 17, 2021, Borislav Petkov wrote: > > > IOW, you have c_bit so your valid address space is [0 .. c_bit-1] no? > > > > I haven't found anything in the GHCB that dictates that MAXPHYADDR == > > C_BIT-1, > > or more specifically that MAXPHYADDR == C_BIT - PhysAddrReduction. E.g. > > AFAICT, > > a VMM could do C_BIT=47, MAXPHYADDR=36, PhysAddrReduction=0, and that would > > be > > allowed by the GHCB. > > > > Forcing "c->x86_phys_bits = c_bit - 1" doesn't seem like it would break > > anything, > > but it's also technically wrong. > > On the other hand, "C_BIT=47, MAXPHYADDR=36, PhysAddrReduction=0" would mean > the > C-bit is an illegal PA bit from the guest's perspective. That's rather > nonsensical, but also not technically disallowed by the APM or GHCB specs.
The C-bit location on Rome is 47 but it's 51 on Milan. So we already have a C-bit that is an illegal PA bit.
