Re: [PATCH 1/6] lib/scatterlist: add sg_set_dma_addr() function

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

Am 12.03.20 um 15:19 schrieb Jason Gunthorpe:
> On Thu, Mar 12, 2020 at 03:47:29AM -0700, Christoph Hellwig wrote:
>> On Thu, Mar 12, 2020 at 11:31:35AM +0100, Christian König wrote:
>>> But how should we then deal with all the existing interfaces which already
>>> take a scatterlist/sg_table ?
>>>
>>> The whole DMA-buf design and a lot of drivers are build around
>>> scatterlist/sg_table and to me that actually makes quite a lot of sense.
>>>
>> Replace them with a saner interface that doesn't take a scatterlist.
>> At very least for new functionality like peer to peer DMA, but
>> especially this code would also benefit from a general move away
>> from the scatterlist.
> If dma buf can do P2P I'd like to see support for consuming a dmabuf
> in RDMA.

That would indeed be awesome.

> Looking at how.. there is an existing sgl based path starting
> from get_user_pages through dma map to the drivers. (ib_umem)
>
> I can replace the driver part with something else (dma_sg), but not
> until we get a way to DMA map pages directly into that something
> else..
>
> The non-page scatterlist is also a big concern for RDMA as we have
> drivers that want the page list, so even if we did as this series
> contemplates I'd have still have to split the drivers and create the
> notion of a dma-only SGL.

Yeah that's my concern as well. For GPU drivers I don't think we need 
the struct pages anywhere, but that might not be true for others.

>>> I mean we could come up with a new structure for this, but to me that just
>>> looks like reinventing the wheel. Especially since drivers need to be able
>>> to handle both I/O to system memory and I/O to PCIe BARs.
>> The structure for holding the struct page side of the scatterlist is
>> called struct bio_vec, so far mostly used by the block and networking
>> code.
> I haven't used bio_vecs before, do they support chaining like SGL so
> they can be very big? RDMA dma maps gigabytes of memory
>
>> The structure for holding dma addresses doesn't really exist
>> in a generic form, but would be an array of these structures:
>>
>> struct dma_sg {
>> 	dma_addr_t	addr;
>> 	u32		len;
>> };
> Same question, RDMA needs to represent gigabytes of pages in a DMA
> list, we will need some generic way to handle that. I suspect GPU has
> a similar need? Can it be accomidated in some generic dma_sg?

Yes, we easily have ranges of >1GB. So I would certainly say u64 for the 
len here.

> So I'm guessing the path forward is something like
>
>   - Add some generic dma_sg data structure and helper
>   - Add dma mapping code to go from pages to dma_sg
>   - Rework RDMA to use dma_sg and the new dma mapping code
>   - Rework dmabuf to support dma mapping to a dma_sg
>   - Rework GPU drivers to use dma_sg
>   - Teach p2pdma to generate a dma_sg from a BAR page list
>   - This series
>
> ?

Sounds pretty much like a plan to me, but unfortunately like a rather 
huge one.

Because of this and cause I don't know if all drivers can live with 
dma_sg I'm not sure if we shouldn't have the switch from scatterlist to 
dma_sg separately to this peer2peer work.

Christian.

>
> Jason

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