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

[Liam Mark]

On Tue, 6 Feb 2018, Alexey Skidanov wrote:

> 
> 
> On 02/07/2018 01:56 AM, Laura Abbott wrote:
> > On 01/31/2018 10:10 PM, Alexey Skidanov wrote:
> >>
> >> On 01/31/2018 03:00 PM, Greg KH wrote:
> >>> On Wed, Jan 31, 2018 at 02:03:42PM +0200, Alexey Skidanov wrote:
> >>>> Any driver may access shared buffers, created by ion, using
> >>>> dma_buf_vmap and
> >>>> dma_buf_vunmap dma-buf API that maps/unmaps previosuly allocated
> >>>> buffers into
> >>>> the kernel virtual address space. The implementation of these API is
> >>>> missing in
> >>>> the current ion implementation.
> >>>>
> >>>> Signed-off-by: Alexey Skidanov <alexey.skidanov@xxxxxxxxx>
> >>>> ---
> >>>
> >>> No review from any other Intel developers? :(
> >> Will add.
> >>>
> >>> Anyway, what in-tree driver needs access to these functions?
> >> I'm not sure that there are the in-tree drivers using these functions
> >> and ion as> buffer exporter because they are not implemented in ion :)
> >> But there are some in-tre> drivers using these APIs (gpu drivers) with
> >> other buffer exporters.
> > 
> > It's still not clear why you need to implement these APIs.
> How the importing kernel module may access the content of the buffer? :)
> With the current ion implementation it's only possible by dma_buf_kmap,
> mapping one page at a time. For pretty large buffers, it might have some
> performance impact.
> (Probably, the page by page mapping is the only way to access large
> buffers on 32 bit systems, where the vmalloc range is very small. By the
> way, the current ion dma_map_kmap doesn't really map only 1 page at a
> time - it uses the result of vmap() that might fail on 32 bit systems.)
> 
> > Are you planning to use Ion with GPU drivers? I'm especially
> > interested in this if you have a non-Android use case.
> Yes, my use case is the non-Android one. But not with GPU drivers.
> > 
> > Thanks,
> > Laura
> 
> Thanks,
> Alexey

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.

Based on discussions at LPC here is what was proposed:
- #1 Add support to ION for dma_buf_vmap and dma_buf_vunmap
- #2 Move any existing ION clients over from using dma_buf_kmap to 
dma_buf_vmap
- #3 Deprecate support in ION for dma_buf_kmap?
- #4 Make the above performance optimization to 
ion_dma_buf_begin_cpu_access to remove the creation of a kernel mapping.

Thoughts?

Liam

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