Re: [PATCH] x86/cpu/AMD: Adjust x86_phys_bits to account for reduced PA in SEV-* guests

[Sean Christopherson]

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.