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.
