Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL

[Logan Gunthorpe]

On 2020-12-09 6:22 p.m., Dan Williams wrote:
> On Mon, Nov 9, 2020 at 8:47 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 2020-11-09 2:12 a.m., Christoph Hellwig wrote:
>>> On Fri, Nov 06, 2020 at 10:00:25AM -0700, Logan Gunthorpe wrote:
>>>> We make use of the top bit of the dma_length to indicate a P2PDMA
>>>> segment.
>>>
>>> I don't think "we" can.  There is nothing limiting the size of a SGL
>>> segment.
>>
>> Yes, I expected this would be the unacceptable part. Any alternative ideas?
> 
> Why is the SG_P2PDMA_FLAG needed as compared to checking the SGL
> segment-pages for is_pci_p2pdma_page()?

Because the DMA and page segments in the SGL aren't necessarily aligned...

The IOMMU implementations can coalesce multiple pages into fewer DMA
address ranges, so the page pointed to by sg->page_link may not be the
one that corresponds to the address in sg->dma_address for a given segment.

If that makes sense -- it's not the easiest thing to explain.

Logan