linux-kernel - RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

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Date:   Fri, 2 Sep 2022 15:33:20 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Borislav Petkov <bp@...en8.de>
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Subject: RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

[AMD Official Use Only - General]

Hello Boris,

>> As I mentioned earlier, this is computing an index into a 4K page 
>> within a hugepage mapping, therefore, though pte_index() works for 2M 
>> pages, but pmd_index() will not work for 1G pages.

>Why not? What exactly do you need to get here?

>So the way I understand it is, you want to map the faulting address to a RMP entry. And that is either the 2M PMD entry when the page is a 1G one and the 4K PTE entry when the page is a 2M one?

>Why doesn't pmd_index() work?

Yes we want to map the faulting address to a RMP entry, but hugepage entries in RMP table are basically subpage 4K entries. So it is a 4K entry when the page is a 2M one 
and also a 4K entry when the page is a 1G one.

That's why the computation to get a 4K page index within a 2M/1G hugepage mapping is required.

>Also, why isn't the lookup function's signature:

>int snp_lookup_rmpentry(unsigned long address, int *level)

>and all that logic to do the conversion to a PFN also not in it?

Thanks,
Ashish