From: Robin Murphy <robin.murphy@arm.com>
To: Jason Gunthorpe <jgg@nvidia.com>, Christoph Hellwig <hch@infradead.org>
Cc: "Leon Romanovsky" <leon@kernel.org>,
kvm@vger.kernel.org, linux-rdma@vger.kernel.org,
"Daniel Vetter" <daniel.vetter@ffwll.ch>,
"Oded Gabbay" <ogabbay@kernel.org>,
"Cornelia Huck" <cohuck@redhat.com>,
dri-devel@lists.freedesktop.org,
"Christian König" <christian.koenig@amd.com>,
linaro-mm-sig@lists.linaro.org,
"Alex Williamson" <alex.williamson@redhat.com>,
"Dan Williams" <dan.j.williams@intel.com>,
"Maor Gottlieb" <maorg@nvidia.com>,
"Sumit Semwal" <sumit.semwal@linaro.org>,
linux-media@vger.kernel.org
Subject: Re: [PATCH v2 4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf
Date: Wed, 7 Sep 2022 17:31:14 +0100 [thread overview]
Message-ID: <6a42a8bc-4e2e-4502-3e7b-1a616dfee351@arm.com> (raw)
In-Reply-To: <Yxi3cFfs0SA4XWJw@nvidia.com>
On 2022-09-07 16:23, Jason Gunthorpe wrote:
> On Wed, Sep 07, 2022 at 07:29:58AM -0700, Christoph Hellwig wrote:
>> On Wed, Sep 07, 2022 at 09:33:11AM -0300, Jason Gunthorpe wrote:
>>> Yes, you said that, and I said that when the AMD driver first merged
>>> it - but it went in anyhow and now people are using it in a bunch of
>>> places.
>>
>> drm folks made up their own weird rules, if they internally stick
>> to it they have to listen to it given that they ignore review comments,
>> but it violates the scatterlist API and has not business anywhere
>> else in the kernel. And yes, there probably is a reason or two why
>> the drm code is unusually error prone.
>
> That may be, but it is creating problems if DRM gets to do X crazy
> thing and nobody else can..
>
> So, we have two issues here
>
> 1) DMABUF abuses the scatter list, but this is very constrainted we have
> this weird special "DMABUF scatterlist" that is only touched by DMABUF
> importers. The imports signal that they understand the format with
> a flag. This is ugly and would be nice to clean to a dma mapped
> address list of some sort.
>
> I spent alot of time a few years ago removing driver touches of
> the SGL and preparing the RDMA stack to do this kind of change, at
> least.
>
> 2) DMABUF abuses dma_map_resource() for P2P and thus doesn't work in
> certain special cases.
FWIW, dma_map_resource() *is* for P2P in general. The classic case of
one device poking at another's registers that was the original
motivation is a standalone DMA engine reading/writing a peripheral
device's FIFO, so the very similar inter-device doorbell signal is
absolutely in scope too; VRAM might be a slightly greyer area, but if
it's still not page-backed kernel memory then I reckon that's fair game.
The only trouble is that it's not geared for *PCI* P2P when that may or
may not happen entirely upstream of IOMMU translation.
Robin.
next prev parent reply other threads:[~2022-09-07 16:31 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-31 23:12 [PATCH v2 0/4] Allow MMIO regions to be exported through dma-buf Jason Gunthorpe
2022-08-31 23:12 ` [PATCH v2 1/4] dma-buf: Add dma_buf_try_get() Jason Gunthorpe
2022-09-01 7:55 ` Christian König
2022-09-06 16:44 ` Jason Gunthorpe
2022-09-06 17:52 ` Christian König
2022-08-31 23:12 ` [PATCH v2 2/4] vfio: Add vfio_device_get() Jason Gunthorpe
2022-08-31 23:12 ` [PATCH v2 3/4] vfio_pci: Do not open code pci_try_reset_function() Jason Gunthorpe
2022-08-31 23:12 ` [PATCH v2 4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf Jason Gunthorpe
[not found] ` <YxcYGzPv022G2vLm@infradead.org>
2022-09-06 10:38 ` Christian König
2022-09-06 11:48 ` Jason Gunthorpe
2022-09-06 12:34 ` Oded Gabbay
2022-09-06 17:59 ` Jason Gunthorpe
2022-09-06 19:44 ` Oded Gabbay
[not found] ` <YxiJJYtWgh1l0wxg@infradead.org>
2022-09-07 12:33 ` Jason Gunthorpe
[not found] ` <Yxiq5sjf/qA7xS8A@infradead.org>
2022-09-07 14:46 ` Oded Gabbay
2022-09-07 15:23 ` Jason Gunthorpe
[not found] ` <Yxi5h09JAzIo4Kh8@infradead.org>
2022-09-07 16:12 ` Jason Gunthorpe
[not found] ` <Yxs+k6psNfBLDqdv@infradead.org>
2022-09-09 14:09 ` Jason Gunthorpe
2022-09-07 16:31 ` Robin Murphy [this message]
2022-09-07 16:47 ` Jason Gunthorpe
2022-09-07 17:03 ` Dan Williams
2022-09-07 15:01 ` Robin Murphy
[not found] ` <YxiIkh/yeWQkZ54x@infradead.org>
2022-09-07 15:08 ` Christian König
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