Re: [PATCH v2 0/3] udmabuf: Sync to attached devices

["Christian König" <christian.koenig@amd.com>]

On 14.08.25 18:10, Andrew Davis wrote:
> Hello all,
> 
> This series makes it so the udmabuf will sync the backing buffer
> with the set of attached devices as required for DMA-BUFs when
> doing {begin,end}_cpu_access.

Yeah the reason why we didn't do that is that this doesn't even work 100% reliable in theory. So this patchset here might make your use case work but is a bit questionable in general.

udmabuf is about turning a file descriptor created by memfd_create() into a DMA-buf. Mapping that memory can happen through the memfd as well and so it is perfectly valid to skip the DMA-buf begin_access and end_access callbacks.

Additional to that when CONFIG_DMABUF_DEBUG is enabled the DMA-buf code mangles the page addresses in the sg table to prevent importers from abusing it. That makes dma_sync_sgtable_for_cpu() and dma_sync_sgtable_for_device() on the exporter side crash.

That's the reason why DMA-buf heaps uses a copy of the sg table for calling dma_sync_sgtable_for_cpu()/dma_sync_sgtable_for_device().

It's basically a hack and should be removed, but for this we need to change all clients which is tons of work.

Regards,
Christian.

> 
> Thanks
> Andrew
> 
> Changes for v2:
>  - fix attachment table use-after-free
>  - rebased on v6.17-rc1
> 
> Andrew Davis (3):
>   udmabuf: Keep track current device mappings
>   udmabuf: Sync buffer mappings for attached devices
>   udmabuf: Use module_misc_device() to register this device
> 
>  drivers/dma-buf/udmabuf.c | 133 +++++++++++++++++++-------------------
>  1 file changed, 67 insertions(+), 66 deletions(-)
>