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

[Tom Lendacky]

On 1/7/20 6:04 PM, Sean Christopherson wrote:
> On Tue, Jan 07, 2020 at 05:51:51PM -0600, Tom Lendacky wrote:
>> On 1/7/20 5:31 PM, Sean Christopherson wrote:
>>> AIUI, using phys_bits=48, then the standard scenario is Cbit=47 and some
>>> additional bits 46:M are reserved.  Applying that logic to phys_bits=52,
>>> then Cbit=51 and bits 50:M are reserved, so there's a collision but it's
>>
>> There's no requirement that the C-bit correspond to phys_bits. So, for
>> example, you can have C-bit=51 and phys_bits=48 and so 47:M are reserved.
> 
> But then using blindly using x86_phys_bits would break if the PA bits
> aren't reduced, e.g. C-bit=47 and phys_bits=47. AFAICT, there's no
> requirement that there be reduced PA bits when there is a C-bit.  I'm
> guessing there aren't plans to ship such CPUs, but I don't see anything
> in the APM to prevent such a scenario.

I can add in extra checks to see if C-bit == phys_bits, etc. and adjust
with appropriate limit checking. It's in the init path, so the extra
checks aren't a big deal.

Thanks,
Tom

> 
> Maybe the least painful approach would be to go with a version of this
> patch and add a check that there are indeeded reserved/reduced bits?
> Probably with a WARN_ON_ONCE if the check fails.
>