Re: [PATCH 0/7] KVM: x86: guest MAXPHYADDR and C-bit fixes

From: Tom Lendacky
Date: Thu Jun 24 2021 - 12:36:44 EST

> Here's an explanation of the physical address reduction for bare-metal and
> guest.
> With MSR 0xC001_0010[SMEE] = 0:
> No reduction in host or guest max physical address.
> With MSR 0xC001_0010[SMEE] = 1:
> - Reduction in the host is enumerated by CPUID 0x8000_001F_EBX[11:6],
> regardless of whether SME is enabled in the host or not. So, for example
> on EPYC generation 2 (Rome) you would see a reduction from 48 to 43.
> - There is no reduction in physical address in a legacy guest (non-SEV
> guest), so the guest can use a 48-bit physical address
> - There is a reduction of only the encryption bit in an SEV guest, so
> the guest can use up to a 47-bit physical address. This is why the
> Qemu command line sev-guest option uses a value of 1 for the
> "reduced-phys-bits" parameter.

The guest statements all assume that NPT is enabled.