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

[Logan Gunthorpe]

On 2020-03-12 8:19 a.m., Jason Gunthorpe wrote:
> 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. 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.
> 
>>> 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

bio_vec's themselves don't support chaining... In the block layer they
are used in a struct bio which handles chaining, splitting and other
features. Each bio, though, has a limit of 256 segments to avoid higher
order allocations. Depending on your use case, you could reuse bios or
write your own container to chain bio_vecs.

>> 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;
>> };


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

I'd probably avoid the u64 here and leave space for some flags or
something. If you have >1GB to map you can always just have mulitple
segments. With 4GB per segment and 256 segments per page, a page of DMA
sgs can easily map 1TB of memory in a single call and with chaining or
larger allocations you can extend that further, if needed.

Logan