Re: [PATCH v3 08/20] iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg

From: Jason Gunthorpe
Date: Tue Sep 28 2021 - 15:15:53 EST

On Thu, Sep 16, 2021 at 05:40:48PM -0600, Logan Gunthorpe wrote:
> When a PCI P2PDMA page is seen, set the IOVA length of the segment
> to zero so that it is not mapped into the IOVA. Then, in finalise_sg(),
> apply the appropriate bus address to the segment. The IOVA is not
> created if the scatterlist only consists of P2PDMA pages.
> A P2PDMA page may have three possible outcomes when being mapped:
> 1) If the data path between the two devices doesn't go through
> the root port, then it should be mapped with a PCI bus address
> 2) If the data path goes through the host bridge, it should be mapped
> normally with an IOMMU IOVA.
> 3) It is not possible for the two devices to communicate and thus
> the mapping operation should fail (and it will return -EREMOTEIO).
> Similar to dma-direct, the sg_dma_mark_pci_p2pdma() flag is used to
> indicate bus address segments. On unmap, P2PDMA segments are skipped
> over when determining the start and end IOVA addresses.
> With this change, the flags variable in the dma_map_ops is set to
> DMA_F_PCI_P2PDMA_SUPPORTED to indicate support for P2PDMA pages.
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> ---
> drivers/iommu/dma-iommu.c | 68 +++++++++++++++++++++++++++++++++++----
> 1 file changed, 61 insertions(+), 7 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>