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:   Wed, 10 Aug 2022 22:00:57 +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,

>> >You need to elaborate more here: a RMP fault can happen and then the 
>> >page can get unmapped? What is the exact scenario here?
>>
>> Yes, if the page gets unmapped while the RMP fault was being handled, 
>> will add more explanation here.

>So what's the logic here to return 1, i.e., retry?

>Why should a fault for a page that gets unmapped be retried? The fault in that case should be ignored, IMO. It'll have the same effect to return from do_user_addr_fault() there, without splitting but you need to have a separate return value >definition so that it is clear what needs to happen. And that return value should be != 0 so that the current check still works.

if (!pte || !pte_present(*pte))
                return 1;

This is more like a sanity check and returning 1 will cause the fault handler to return and ignore the fault for current #PF case.
If the page got unmapped, the fault will not happen again and there will be no retry, so the fault in this case is
being ignored.
The other case where 1 is returned is RMP table lookup failure, in that case the faulting process is being terminated,
that resolves the fault. 

>> Actually, the above computes an index into the RMP table.

>What index in the RMP table?

>> It is basically an index into the 4K page within the hugepage mapped 
>> in the RMP table or in other words an index into the RMP table entry 
>> for 4K page(s) corresponding to a hugepage.

>So pte_index(address) and for 1G pages, pmd_index(address).

>So no reinventing the wheel if we already have helpers for that.

Yes that makes sense and pte_index(address) is exactly what is
required for 2M hugepages.

Will use pte_index() for 2M pages and pmd_index() for 1G pages. 

>> It is mainly a wrapper around__split_huge_pmd() for SNP use case where 
>> the host hugepage is split to be in sync with the RMP table.

>I see what it is. And I'm saying this looks wrong. You're enforcing page splitting to be a valid thing to do only for SEV machines. Why?

>Why is

>        if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
>                return VM_FAULT_SIGBUS;

>there at all?

>This is generic code you're touching - not arch/x86/.

Ok, so you are suggesting that we remove this check and simply keep this function wrapping around __split_huge_pmd(). 
This becomes a generic utility function. 

Thanks,
Ashish