Am 2021-06-22 um 11:29 a.m. schrieb Christian König:

Am 22.06.21 um 17:23 schrieb Jason Gunthorpe:

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On Tue, Jun 22, 2021 at 02:23:03PM +0200, Christian König wrote:

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Am 22.06.21 um 14:01 schrieb Jason Gunthorpe:

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On Tue, Jun 22, 2021 at 11:42:27AM +0300, Oded Gabbay wrote:

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On Tue, Jun 22, 2021 at 9:37 AM Christian König
[off-list ref] wrote:

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Am 22.06.21 um 01:29 schrieb Jason Gunthorpe:

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On Mon, Jun 21, 2021 at 10:24:16PM +0300, Oded Gabbay wrote:

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Another thing I want to emphasize is that we are doing p2p only
through the export/import of the FD. We do *not* allow the user to
mmap the dma-buf as we do not support direct IO. So there is no
access
to these pages through the userspace.

Arguably mmaping the memory is a better choice, and is the
direction
that Logan's series goes in. Here the use of DMABUF was
specifically
designed to allow hitless revokation of the memory, which this
isn't
even using.

The major problem with this approach is that DMA-buf is also used
for
memory which isn't CPU accessible.

That isn't an issue here because the memory is only intended to be
used with P2P transfers so it must be CPU accessible.

No, especially P2P is often done on memory resources which are not even
remotely CPU accessible.

That is a special AMD thing, P2P here is PCI P2P and all PCI memory is
CPU accessible.

No absolutely not. NVidia GPUs work exactly the same way.

And you have tons of similar cases in embedded and SoC systems where
intermediate memory between devices isn't directly addressable with
the CPU.

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So you are taking the hit of very limited hardware support and
reduced
performance just to squeeze into DMABUF..

You still have the issue that this patch is doing all of this P2P
stuff wrong - following the already NAK'd AMD approach.

Well that stuff was NAKed because we still use sg_tables, not
because we
don't want to allocate struct pages.

sg lists in general.
 

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The plan is to push this forward since DEVICE_PRIVATE clearly can't
handle
all of our use cases and is not really a good fit to be honest.

IOMMU is now working as well, so as far as I can see we are all good
here.

How? Is that more AMD special stuff?

No, just using the dma_map_resource() interface.

We have that working on tons of IOMMU enabled systems.

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This patch series never calls to the iommu driver, AFAICT.

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I'll go and read Logan's patch-set to see if that will work for us in
the future. Please remember, as Daniel said, we don't have struct
page
backing our device memory, so if that is a requirement to connect to
Logan's work, then I don't think we will want to do it at this point.

It is trivial to get the struct page for a PCI BAR.

Yeah, but it doesn't make much sense. Why should we create a struct
page for
something that isn't even memory in a lot of cases?

Because the iommu and other places need this handle to setup their
stuff. Nobody has yet been brave enough to try to change those flows
to be able to use a physical CPU address.

Well that is certainly not true. I'm just not sure if that works with
all IOMMU drivers thought.

Would need to ping Felix when the support for this was merged.

We have been working on IOMMU support for all our multi-GPU memory
mappings in KFD. The PCIe P2P side of this is currently only merged on
our internal branch. Before we can actually use this, we need
CONFIG_DMABUF_MOVE_NOTIFY enabled (which is still documented as
experimental and disabled by default). Otherwise we'll end up pinning
all our VRAM.

I think we'll try to put together an upstream patch series of all our
PCIe P2P support in a few weeks or so. This will include IOMMU mappings,
checking that PCIe P2P is actually possible between two devices, and KFD
topology updates to correctly report those capabilities to user mode.

It will not use struct pages for exported VRAM buffers.

Regards,
  Felix

Regards,
Christian.

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This is why we have a special struct page type just for PCI BAR
memory.

Jason

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