linux-kernel - Re: [PATCH v9 3/6] KVM: arm64: Block cacheable PFNMAP mapping

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Message-ID: <4b06b163-e1ce-4c20-b878-4593bc86bf53@redhat.com>
Date: Fri, 4 Jul 2025 14:21:32 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>, Ankit Agrawal <ankita@...dia.com>
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Subject: Re: [PATCH v9 3/6] KVM: arm64: Block cacheable PFNMAP mapping

On 30.06.25 14:25, Jason Gunthorpe wrote:
> On Mon, Jun 30, 2025 at 01:56:43AM +0000, Ankit Agrawal wrote:
>>> Sorry for the drive-by comment, but I was looking at this old series from
>>> Paolo (look at the cover letter and patch 5):
>>>
>>> https://lore.kernel.org/r/20250109133817.314401-1-pbonzini@redhat.com
>>>
>>> in which he points out that the arm64 get_vma_page_shift() function
>>> incorrectly assumes that a VM_PFNMAP VMA is physically contiguous, which
>>> may not be the case if a driver calls remap_pfn_range() to mess around
>>> with mappings within the VMA. I think that implies that the optimisation
>>> in 2aa53d68cee6 ("KVM: arm64: Try stage2 block mapping for host device
>>> MMIO") is unsound.
>>
>> Hm yeah, that does seem problematic. Perhaps we need a new
>> vma flag that could help the driver communicate to the KVM that the
>> mapping is contiguous and it can go ahead with the optimization?
>> E.g. something similar to VM_ALLOW_ANY_UNCACHED.
> 
> I think Paolo has the right direction - remove any attempts by KVM to
> expand contiguity, it should only copy the primary's PTEs and rely on
> the primary to discover contiguity. No new flags.

100%

-- 
Cheers,

David / dhildenb