On Wed, Dec 9, 2020 at 6:07 PM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote: > > > > 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. It does... Did someone already grab, or did you already consider the 3rd available bit in page_link? AFAICS only SG_CHAIN and SG_END are reserved. However, if you have a CONFIG_64BIT dependency for user-directed p2pdma that would seem to allow SG_P2PDMA_FLAG to be (0x4) in page_link.
