Re: [RFC PATCH v1 10/18] x86/efi: Access EFI related tables in the clear

[Tom Lendacky]

On 06/08/2016 05:07 AM, Matt Fleming wrote:
> (Sorry for the delay)

No worries, thanks for all the feedback.

> 
> On Thu, 26 May, at 08:45:58AM, Tom Lendacky wrote:
>>
>> The patch in question is patch 6/18 where PAGE_KERNEL is changed to
>> include the _PAGE_ENC attribute (the encryption mask). This now
>> makes FIXMAP_PAGE_NORMAL contain the encryption mask while
>> FIXMAP_PAGE_IO does not. In this way, anything mapped using the
>> early_ioremap call won't be mapped encrypted.
> 
> There are semantics attached to early_ioremap() that do not apply in
> this case; that you're mapping an MMIO region but for EFI we just care
> about noting where the firmware (not the kernel) populated the region
> with data. Similar problems exist for other early boot code such as
> the devicetree stuff.
> 
> early_ioremap() is not the answer.
> 
> What you really want is just some way to distinguish kernel-owned
> regions from those owned by "somebody else".
> 
> I have no problem updating early_memremap() to take a @flags argument
> to make that distinction, provided that the naming is generic and not
> tied to AMD's SME technology via an "sme" prefix/suffix.

So maybe something along the lines of an enum that would have entries
(initially) like KERNEL_DATA (equal to zero) and EFI_DATA. Others could
be added later as needed.

Would you then want to allow the protection attributes to be updated
by architecture specific code through something like a __weak function?
In the x86 case I can add this function as a non-SME specific function
that would initially just have the SME-related mask modification in it.

Thanks,
Tom

> 
> And making it generic should allow it to be easily sprinkled into the
> shared architecture code in drivers/firmware/efi/ without issue.
> 
> I'm going to follow up with some additional comments/questions on
> PATCH 10.
>