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

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Message-ID: <SN6PR12MB2767C4C296281D25306885A78E7D9@SN6PR12MB2767.namprd12.prod.outlook.com>
Date:   Sat, 3 Sep 2022 17:30:28 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Boris 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,

>>So essentially we want to map the faulting address to a RMP entry, 
>>considering the fact that a 2M host hugepage can be mapped as 4K RMP table entries and 1G host hugepage can be mapped as 2M RMP table entries.

>So something's seriously confusing or missing here because if you fault on a 2M host page and the underlying RMP entries are 4K then you can use pte_index().

>If the host page is 1G and the underlying RMP entries are 2M, pmd_index() should work here too.

>But this piecemeal back'n'forth doesn't seem to resolve this so I'd like to ask you pls to sit down, take your time and give a detailed example of the two possible cases and what the difference is between pte_/pmd_index and your way. Feel free to >add actual debug output and paste it here.

There is 1 64-bit RMP entry for every physical 4k page of DRAM, so essentially every 4K page of DRAM is represented by a RMP entry.

So even if host page is 1G and underlying (smashed/split) RMP entries are 2M, the RMP table entry has to be indexed to a 4K entry
corresponding to that.

If it was simply a 2M entry in the RMP table, then pmd_index() will work correctly.

Considering the following example: 

address = 0x40200000; 
level = PG_LEVEL_1G;
pfn  = 0x40000;
pfn |= pmd_index(address);
This will give the RMP table index as 0x40001.
And it will work if the RMP table entry was simply a 2MB entry, but we need to map this further to its corresponding 4K entry.

With the same example as above: 
level = PG_LEVEL_1G;
mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
pfn |= (address >> PAGE_SHIFT) & mask;
This will give the RMP table index as 0x40200.       
Which is the correct RMP table entry for a 2MB smashed/split 1G page mapped further to its corresponding 4K entry.

Hopefully this clarifies why pmd_index() can't be used here.

Thanks,
Ashish