Re: [PATCH v6 9/9] vfio/pci: Permanently revoke a DMABUF on request

[Christian König]

On 9/15/26 09:20, Leon Romanovsky wrote:
> On Mon, Sep 14, 2026 at 01:13:36PM +0100, Matt Evans wrote:
>> Hi Leon,
>>
>> On 14/09/2026 12:54, Leon Romanovsky wrote:
>>> On Mon, Sep 14, 2026 at 08:36:47AM -0300, Jason Gunthorpe wrote:
>>>> On Sun, Sep 13, 2026 at 07:52:44PM +0300, Leon Romanovsky wrote:
>>>>> On Fri, Sep 11, 2026 at 10:41:57PM +0100, Matt Evans wrote:
>>>>>> Expand the VFIO DMABUF revocation state to three states:
>>>>>> Not revoked, temporarily revoked, and permanently revoked.
>>>>>
>>>>> The thing is that "temporarily revoked" is actually the standard
>>>>> invalidate_mappings/move_notify mechanism of DMABUF, which wasn't good
>>>>> for VFIO.
>>>>
>>>> I think temporarily revokes here means it is revoked from a dmabuf
>>>> perspective
>>>
>>> My guess is that this is more of a "change owner" operation than a
>>> revoke operation.
>>>
>>> The main issue here is that we have to guess the semantics instead of
>>> having a properly named and documented operation.
>>
>> Apologies if the cover letter for the series and patch commit message (which
>> cover this) are unclear about the motivations and semantics.  On the commit
>> message, can you suggest clarifications:
>>
>> "This is useful for lifecycle management, to reclaim VFIO PCI BAR
>> ranges previously delegated to a subordinate client process: by
>> revoking, the driver process can ensure that the loaned resources are
>> made inaccessible when the client is deemed "done".  The original
>> DMABUF is defunct, and BAR resources can then be safely re-exported
>> for use by new clients."
>>
>> Given what I'll explain below, do give suggestions please.  There is more
>> context in the cover letter (the volume of which I didn't think appropriate
>> for the commit message).
> 
> 1. Do not mix "driver" and "client" in the same description. For a
> non-native English speaker, "driver" has a very specific meaning in the
> context of the Linux kernel.

In the context of DMA-buf it has also proven vital to clearly use the terms importer and exporter to describe the different roles a driver can have.

I can't count how often there was confusion because people (me included) just used "driver" and it wasn't clear which role was meant.

> 
> 2. Explain the lifecycle in the commit message, and why "revoke", which
> is effectively what the importer does, is not sufficient.
> 
> 3. The more you put in the cover letter, the less likely people are to
> read it.
> 
> 4. Commit messages should describe the patches themselves, since they
> are what remains visible in the git log, unlike the cover letter.
> 
> 5. I would call what you describe as "temporarily revoke" is actually "reclaim".

+1

Regards,
Christian.

> 
> Thanks