Re: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and dma_buf_vunmap

[Liam Mark <lmark@codeaurora.org>]

On Tue, 18 Dec 2018, Alexey Skidanov wrote:

> >>> I was wondering if we could re-open the discussion on adding support to 
> >>> ION for dma_buf_vmap.
> >>> It seems like the patch was not taken as the reviewers wanted more 
> >>> evidence of an upstream use case.
> >>>
> >>> Here would be my upstream usage argument for including dma_buf_vmap 
> >>> support in ION.
> >>>
> >>> Currently all calls to ion_dma_buf_begin_cpu_access result in the creation 
> >>> of a kernel mapping for the buffer, unfortunately the resulting call to 
> >>> alloc_vmap_area can be quite expensive and this has caused a performance 
> >>> regression for certain clients when they have moved to the new version of 
> >>> ION.
> >>>
> >>> The kernel mapping is not actually needed in ion_dma_buf_begin_cpu_access, 
> >>> and generally isn't needed by clients. So if we remove the creation of the 
> >>> kernel mapping in ion_dma_buf_begin_cpu_access and only create it when 
> >>> needed we can speed up the calls to ion_dma_buf_begin_cpu_access.
> >>>
> >>> An additional benefit of removing the creation of kernel mappings from 
> >>> ion_dma_buf_begin_cpu_access is that it makes the ION code more secure.
> >>> Currently a malicious client could call the DMA_BUF_IOCTL_SYNC IOCTL with 
> >>> flags DMA_BUF_SYNC_END multiple times to cause the ION buffer kmap_cnt to 
> >>> go negative which could lead to undesired behavior.
> >>>
> >>> One disadvantage of the above change is that a kernel mapping is not 
> >>> already created when a client calls dma_buf_kmap. So the following 
> >>> dma_buf_kmap contract can't be satisfied.
> >>>
> >>> /**
> >>> * dma_buf_kmap - Map a page of the buffer object into kernel address 
> >>> space. The
> >>> * same restrictions as for kmap and friends apply.
> >>> * @dmabuf:	[in]	buffer to map page from.
> >>> * @page_num:	[in]	page in PAGE_SIZE units to map.
> >>> *
> >>> * This call must always succeed, any necessary preparations that might 
> >>> fail
> >>> * need to be done in begin_cpu_access.
> >>> */
> >>>
> >>> But hopefully we can work around this by moving clients to dma_buf_vmap.
> >> I think the problem is with the contract. We can't ensure that the call
> >> is always succeeds regardless the implementation - any mapping might
> >> fail. Probably this is why  *all* clients of dma_buf_kmap() check the
> >> return value (so it's safe to return NULL in case of failure).
> >>
> > 
> > I think currently the call to dma_buf_kmap will always succeed since the 
> > DMA-Buf contract requires that the client first successfully call 
> > dma_buf_begin_cpu_access(), and if dma_buf_begin_cpu_access() succeeds 
> > then dma_buf_kmap will succeed.
> > 
> >> I would suggest to fix the contract and to keep the dma_buf_kmap()
> >> support in ION.
> > 
> > I will leave it to the DMA-Buf maintainers as to whether they want to 
> > change their contract.
> > 
> > Liam
> > 
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> > a Linux Foundation Collaborative Project
> > 
> 
> Ok. We need the list of the clients using the ION in the mainline tree.
> 

Looks to me like the only functions which might be calling 
dma_buf_kmap/dma_buf_kunmap on ION buffers are 
tegra_bo_kmap/tegra_bo_kunmap, I assume Tegra is used in some Android 
automotive products.

Looks like these functions could be moved over to using 
dma_buf_vmap/dma_buf_vunmap but it wouldn't be very clean and would add a 
performance hit.

Liam

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project