linux-kernel - Re: [Linaro-mm-sig] [PATCH v8 06/11] dma-buf: provide phys

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Message-ID: <3053398d-94d8-42fa-aedc-927746375521@amd.com>
Date: Thu, 20 Nov 2025 08:03:09 +0100
From: Christian König <christian.koenig@....com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, Logan Gunthorpe
 <logang@...tatee.com>, Jens Axboe <axboe@...nel.dk>,
 Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Marek Szyprowski <m.szyprowski@...sung.com>,
 Jason Gunthorpe <jgg@...pe.ca>, Andrew Morton <akpm@...ux-foundation.org>,
 Jonathan Corbet <corbet@....net>, Sumit Semwal <sumit.semwal@...aro.org>,
 Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Ankit Agrawal <ankita@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
 Shameer Kolothum <skolothumtho@...dia.com>, Kevin Tian
 <kevin.tian@...el.com>, Alex Williamson <alex@...zbot.org>,
 Krishnakant Jaju <kjaju@...dia.com>, Matt Ochs <mochs@...dia.com>,
 linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org, iommu@...ts.linux.dev, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-media@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
 kvm@...r.kernel.org, linux-hardening@...r.kernel.org,
 Alex Mastro <amastro@...com>, Nicolin Chen <nicolinc@...dia.com>
Subject: Re: [Linaro-mm-sig] [PATCH v8 06/11] dma-buf: provide phys_vec to
 scatter-gather mapping routine

On 11/19/25 17:33, Leon Romanovsky wrote:
> On Wed, Nov 19, 2025 at 03:53:30PM +0100, Christian König wrote:
> 
> <...>
> 
>>>>>>> +struct sg_table *dma_buf_map(struct dma_buf_attachment *attach,
>>>>>>
>>>>>> That is clearly not a good name for this function. We already have overloaded the term *mapping* with something completely different.
>>>>>
>>>>> This function performs DMA mapping, so what name do you suggest instead of dma_buf_map()?
>>>>
>>>> Something like dma_buf_phys_vec_to_sg_table(). I'm not good at naming either.
>>>
>>> Can I call it simply dma_buf_mapping() as I plan to put that function in dma_buf_mapping.c
>>> file per-your request.
>>
>> No, just completely drop the term "mapping" here. This is about phys_vector to sg_table conversion and nothing else.
> 
> In order to progress, I renamed these functions to be
> dma_buf_phys_vec_to_sgt() and dma_buf_free_sgt(), and put everything in dma_buf_mapping.c file.

Yeah, the problem is I even thought more about it and came to the conclusion that this is still not sufficient for an rb or an Ack-by.

A core concept of DMA-buf is that the exporter takes care of all the mappings and not the framework.

Calling pci_p2pdma_bus_addr_map(), dma_map_phys() or dma_map_phys() from DMA-buf code is extremely questionable.

That should really be inside VFIO and not DMA-buf code, so to move forward I strongly suggest to either move that into VFIO or the DMA API directly.

Regards,
Christian.

> 
> Thanks