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

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Date:   Tue, 28 Sep 2021 16:15:47 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 08/20] iommu/dma: support PCI P2PDMA pages in
 dma-iommu map_sg

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@...tatee.com>
> ---
>  drivers/iommu/dma-iommu.c | 68 +++++++++++++++++++++++++++++++++++----
>  1 file changed, 61 insertions(+), 7 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason