Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when changing C-bit

Tom Lendacky
2022-02-13T14:50:54Z

On 2/13/22 06:15, Borislav Petkov wrote:
> On Fri, Feb 11, 2022 at 11:27:54AM -0600, Brijesh Singh wrote:
>>> Simply have them always present. They will have !0 values on the
>>> respective guest types and 0 otherwise. This should simplify a lot of
>>> code and another unconditionally present u64 won't be the end of the
>>> world.
>>>
>>> Any other aspect I'm missing?
>>
>> I think that's mostly about it. IIUC, the recommendation is to define a
>> new callback in x86_platform_op. The callback will be invoked
>> unconditionally; The default implementation for this callback is NOP;
>> The TDX and SEV will override with the platform specific implementation.
>> I think we may able to handle everything in one callback hook but having
>> pre and post will be a more desirable. Here is why I am thinking so:
>>
>> * On SNP, the page must be invalidated before clearing the _PAGE_ENC
>> from the page table attribute
>>
>> * On SNP, the page must be validated after setting the _PAGE_ENC in the
>> page table attribute.
>
> Right, we could have a pre- and post- callback, if that would make
> things simpler/clearer.
>
> Also, in thinking further about the encryption mask, we could make it a
> *single*, *global* variable called cc_mask which each guest type sets it
> as it wants to.
>
> Then, it would use it in the vendor-specific encrypt/decrypt helpers
> accordingly and that would simplify a lot of code. And we can get rid of
> all the ifdeffery around it too.
>
> So I think the way to go should be we do the common functionality, I
> queue it on the common tip:x86/cc branch and then SNP and TDX will be
> both based ontop of it.
>
> Thoughts?

I think there were a lot of assumptions that only SME/SEV would set
sme_me_mask and that is used, for example, in the cc_platform_has()
routine to figure out whether we're AMD or Intel. If you go the cc_mask
route, I think we'll need to add a cc_vendor variable that would then be
checked in cc_platform_has(). All other uses of sme_me_mask would need to
be audited to see whether cc_vendor would need to be checked, too.

Thanks,
Tom

>