Re: [PATCH v2] KVM: SVM: Override default MMIO mask if memory encryption is enabled

Sean Christopherson
2020-01-06T23:38:48Z

On Mon, Jan 06, 2020 at 05:14:04PM -0600, Tom Lendacky wrote:
> On 1/6/20 4:49 PM, Sean Christopherson wrote:
> > This doesn't handle the case where x86_phys_bits _isn't_ reduced by SME/SEV
> > on a future processor, i.e. x86_phys_bits==52.
>
> Not sure I follow. If MSR_K8_SYSCFG_MEM_ENCRYPT is set then there will
> always be a reduction in physical addressing (so I'm told).

Hmm, I'm going off APM Vol 2, which states, or at least strongly implies,
that reducing the PA space is optional. Section 7.10.2 is especially
clear on this:

In implementations where the physical address size of the processor is
reduced when memory encryption features are enabled, software must
ensure it is executing from addresses where these upper physical address
bits are 0 prior to setting SYSCFG[MemEncryptionModEn].

But, hopefully the other approach I have in mind actually works, as it's
significantly less special-case code and would naturally handle either
case, i.e. make this a moot point.

Entry on SYSCFG:

3.2.1 System Configuration Register (SYSCFG)

...

MemEncryptionMode. Bit 23. Setting this bit to 1 enables the SME and
SEV memory encryption features.

The SME entry the above links to says:

7.10.1 Determining Support for Secure Memory Encryption

...

Additionally, in some implementations, the physical address size of the
processor may be reduced when memory encryption features are enabled, for
example from 48 to 43 bits. In this case the upper physical address bits are
treated as reserved when the feature is enabled except where otherwise
indicated. When memory encryption is supported in an implementation, CPUID
Fn8000_001F[EBX] reports any physical address size reduction present. Bits
reserved in this mode are treated the same as other page table reserved bits,
and will generate a page fault if found to be non-zero when used for address
translation.

...

7.10.2 Enabling Memory Encryption Extensions

Prior to using SME, memory encryption features must be enabled by setting
SYSCFG MSR bit 23 (MemEncryptionModEn) to 1. In implementations where the
physical address size of the processor is reduced when memory encryption
features are enabled, software must ensure it is executing from addresses where
these upper physical address bits are 0 prior to setting
SYSCFG[MemEncryptionModEn]. Memory encryption is then further controlled via
the page tables.