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

[Skidanov, Alexey]

> -----Original Message-----
> From: Liam Mark [mailto:lmark@xxxxxxxxxxxxxx]
> Sent: Friday, January 04, 2019 19:42
> To: Skidanov, Alexey <alexey.skidanov@xxxxxxxxx>
> Cc: Laura Abbott <labbott@xxxxxxxxxx>; Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>;
> devel@xxxxxxxxxxxxxxxxxxxx; tkjos@xxxxxxxxxxx; rve@xxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; maco@xxxxxxxxxxx; sumit.semwal@xxxxxxxxxx
> Subject: Re: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and
> dma_buf_vunmap
> 
> 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

I'm a little bit confused. Why making the buffer accessible by CPU (mapping the buffer)
and making the content of the buffer valid (coherent) are so tightly coupled in DMA-BUF? 

Alexey