linux-kernel - Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Date: Fri, 10 Jan 2025 21:15:53 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: David Hildenbrand <david@...hat.com>, Jason Gunthorpe <jgg@...dia.com>
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Subject: Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

>>>> This patch solves the problems where it is possible for the kernel to
>>>> have VMAs pointing at cachable memory without causing
>>>> pfn_is_map_memory() to be true, eg DAX memremap cases and CXL/pre-CXL
>>>> devices. This memory is now properly marked as cachable in KVM.
>>>
>>> Does this only imply in worse performance, or does this also affect
>>> correctness? I suspect performance is the problem, correct?
>>
>> Correctness. Things like atomics don't work on non-cachable mappings.
>
> Hah! This needs to be highlighted in the patch description. And maybe
> this even implies Fixes: etc?

Understood. I'll put that in the patch description.

>>> Likely you assume to never end up with COW VM_PFNMAP -- I think it's
>>> possible when doing a MAP_PRIVATE /dev/mem mapping on systems that allow
>>> for mapping /dev/mem. Maybe one could just reject such cases (if KVM PFN
>>> lookup code not already rejects them, which might just be that case IIRC).
>>
>> At least VFIO enforces SHARED or it won't create the VMA.
>>
>> drivers/vfio/pci/vfio_pci_core.c:       if ((vma->vm_flags & VM_SHARED) == 0)
>
> That makes a lot of sense for VFIO.

So, I suppose we don't need to check this? Specially if we only extend the
changes to the following case:
- type is VM_PFNMAP &&
- user mapping is cacheable (MT_NORMAL or MT_NORMAL_TAGGED) &&
- The suggested VM_FORCE_CACHED is set.

- Ankit Agrawal