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

[Liam Mark <lmark@codeaurora.org>]

On Sun, 16 Dec 2018, Alexey Skidanov wrote:

> 
> 
> On 12/16/18 7:20 AM, Liam Mark wrote:
> > 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@intel.com>
> >>>>>> ---
> >>>>>
> >>>>> 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.
> 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

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