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

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Message-ID: <20251120073810.GQ18335@unreal>
Date: Thu, 20 Nov 2025 09:38:10 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Christian König <christian.koenig@....com>
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 Thu, Nov 20, 2025 at 08:03:09AM +0100, Christian König wrote:
> 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.

We got the request to move to DMABUF and agreement a long time ago, in v5.
https://lore.kernel.org/all/aPYrEroyWVOvAu-5@infradead.org/

Thanks

> 
> Regards,
> Christian.
> 
> > 
> > Thanks
>