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

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Message-ID: <c115432c-b63d-4b99-be18-0bf96398e153@amd.com>
Date: Thu, 20 Nov 2025 08:08:27 +0100
From: Christian König <christian.koenig@....com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Leon Romanovsky <leon@...nel.org>, 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>,
 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 20:31, Jason Gunthorpe wrote:
> On Wed, Nov 19, 2025 at 02:42:18PM +0100, Christian König wrote:
> 
>>>>> +	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
>>>>> +		dma->state = kzalloc(sizeof(*dma->state), GFP_KERNEL);
>>>>> +		if (!dma->state) {
>>>>> +			ret = -ENOMEM;
>>>>> +			goto err_free_dma;
>>>>> +		}
>>>>> +
>>>>> +		dma_iova_try_alloc(attach->dev, dma->state, 0, size);
>>>>
>>>> Oh, that is a clear no-go for the core DMA-buf code.
>>>>
>>>> It's intentionally up to the exporter how to create the DMA
>>>> addresses the importer can work with.
>>>
>>> I can't fully understand this remark?
>>
>> The exporter should be able to decide if it actually wants to use
>> P2P when the transfer has to go through the host bridge (e.g. when
>> IOMMU/bridge routing bits are enabled).
> 
> Sure, but this is a simplified helper for exporters that don't have
> choices where the memory comes from.

That is extremely questionable as justification to put that in common DMA-buf code.

> I fully expet to see changes to this to support more use cases,
> including the one above. We should do those changes along with users
> making use of them so we can evaluate what works best.

Yeah, exactly that's my concern.

>> But only take that as Acked-by, I would need at least a day (or
>> week) of free time to wrap my head around all the technical details
>> again. And that is something I won't have before January or even
>> later.
> 
> Sure, it is alot, and I think DRM community in general should come up
> to speed on the new DMA API and how we are pushing to see P2P work
> within Linux.
> 
> So thanks, we can take the Acked-by and progress here. Interested
> parties can pick it up from this point when time allows.

Wait a second. After sleeping a night over it I think my initial take that we really should not put that into common DMA-buf code seems to hold true.

This is the use case for VFIO, but I absolutely want to avoid other drivers from re-using this code until be have more experience with that.

So to move forward I now strongly think we should keep that in VFIO until somebody else comes along and needs that helper.

Regards,
Christian.

> 
> We can also have a mini-community call to give a summary/etc on these
> topics.
> 
> Thanks,
> Jason