Re: [RFC PATCH v3 04/20] x86: Handle reduction in physical address size with SME

[Tom Lendacky]

On 11/15/2016 10:33 AM, Borislav Petkov wrote:
> On Tue, Nov 15, 2016 at 10:06:16AM -0600, Tom Lendacky wrote:
>> Yes, but that doesn't relate to the physical address space reduction.
>>
>> Once the SYS_CFG MSR bit for SME is set, even if the encryption bit is
>> never used, there is a physical reduction of the address space. So when
>> checking whether to adjust the physical address bits I can't rely on the
>> sme_me_mask, I have to look at the MSR.
>>
>> But when I'm looking to decide whether to encrypt or decrypt something,
>> I use the sme_me_mask to decide if that is needed.  If the sme_me_mask
>> is not set then the encrypt/decrypt op shouldn't be performed.
>>
>> I might not be grasping the point you're trying to make...
> 
> Ok, let me try to summarize how I see it. There are a couple of states:
> 
> * CPUID bit in 0x8000001f - that's SME supported
> 
> * Reduction of address space - MSR bit. That could be called "SME
> BIOS-eenabled".
> 
> * SME active. That's both of the above and is sme_me_mask != 0.
> 
> Right?

Correct.

> 
> So you said previously "The feature may be present and enabled even if
> it is not currently active."
> 
> But then you say "active" below
> 
>>> And in patch 12 you have:
>>>
>>> +       /*
>>> +        * If memory encryption is active, the trampoline area will need to
>>> +        * be in un-encrypted memory in order to bring up other processors
>>> +        * successfully.
>>> +        */
>>> +       sme_early_mem_dec(__pa(base), size);
>>> +       sme_set_mem_unenc(base, size);
> 
> and test sme_me_mask. Which makes sense now after having explained which
> hw setting controls what.
> 
> So can we agree on the nomenclature for all the different SME states
> first and use those throughout the code? And hold those states down in
> Documentation/x86/amd-memory-encryption.txt so that it is perfectly
> clear to people looking at the code.

Yup, that sounds good.  I'll update the documentation to clarify the
various states/modes of SME.

> 
> Also, if we need to check those states more than once, we should add
> inline helpers:
> 
> sme_supported()
> sme_bios_enabled()
> sme_active()
> 
> How does that sound?

Sounds good.

Thanks,
Tom

>