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

[Jason Gunthorpe]

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>

Jason