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SubjectRe: [PATCH v7 08/21] iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg
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On 2022-06-15 17:12, 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@deltatee.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> drivers/iommu/dma-iommu.c | 68 +++++++++++++++++++++++++++++++++++----
> 1 file changed, 61 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index f90251572a5d..b01ca0c6a7ab 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -21,6 +21,7 @@
> #include <linux/iova.h>
> #include <linux/irq.h>
> #include <linux/list_sort.h>
> +#include <linux/memremap.h>
> #include <linux/mm.h>
> #include <linux/mutex.h>
> #include <linux/pci.h>
> @@ -1062,6 +1063,16 @@ static int __finalise_sg(struct device *dev, struct scatterlist *sg, int nents,
> sg_dma_address(s) = DMA_MAPPING_ERROR;
> sg_dma_len(s) = 0;
>
> + if (is_pci_p2pdma_page(sg_page(s)) && !s_iova_len) {

Logically, should we not be able to use sg_is_dma_bus_address() here? I
think it should be feasible, and simpler, to prepare the p2p segments
up-front, such that at this point all we need to do is restore the
original length (if even that, see below).

> + if (i > 0)
> + cur = sg_next(cur);
> +
> + pci_p2pdma_map_bus_segment(s, cur);
> + count++;
> + cur_len = 0;
> + continue;
> + }
> +
> /*
> * Now fill in the real DMA data. If...
> * - there is a valid output segment to append to
> @@ -1158,6 +1169,8 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> struct iova_domain *iovad = &cookie->iovad;
> struct scatterlist *s, *prev = NULL;
> int prot = dma_info_to_prot(dir, dev_is_dma_coherent(dev), attrs);
> + struct dev_pagemap *pgmap = NULL;
> + enum pci_p2pdma_map_type map_type;
> dma_addr_t iova;
> size_t iova_len = 0;
> unsigned long mask = dma_get_seg_boundary(dev);
> @@ -1193,6 +1206,35 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> s_length = iova_align(iovad, s_length + s_iova_off);
> s->length = s_length;
>
> + if (is_pci_p2pdma_page(sg_page(s))) {
> + if (sg_page(s)->pgmap != pgmap) {
> + pgmap = sg_page(s)->pgmap;
> + map_type = pci_p2pdma_map_type(pgmap, dev);
> + }

There's a definite code smell here, but per above and below I think we
*should* actually call the new helper instead of copy-pasting half of it.

> +
> + switch (map_type) {
> + case PCI_P2PDMA_MAP_BUS_ADDR:
> + /*
> + * A zero length will be ignored by
> + * iommu_map_sg() and then can be detected

If that is required behaviour then it needs an explicit check in
iommu_map_sg() to guarantee (and document) it. It's only by chance that
__iommu_map() happens to return success for size == 0 *if* all the other
arguments still line up, which is a far cry from a safe no-op.

However, rather than play yet more silly tricks, I think it would make
even more sense to make iommu_map_sg() properly aware and able to skip
direct p2p segments on its own. Once it becomes normal to pass mixed
scatterlists around, it's only a matter of time until one ends up being
handed to a driver which manages its own IOMMU domain, and then what?

> + * in __finalise_sg() to actually map the
> + * bus address.
> + */
> + s->length = 0;
> + continue;
> + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> + /*
> + * Mapping through host bridge should be
> + * mapped with regular IOVAs, thus we
> + * do nothing here and continue below.
> + */
> + break;
> + default:
> + ret = -EREMOTEIO;
> + goto out_restore_sg;
> + }
> + }
> +
> /*
> * Due to the alignment of our single IOVA allocation, we can
> * depend on these assumptions about the segment boundary mask:
> @@ -1215,6 +1257,9 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> prev = s;
> }
>
> + if (!iova_len)
> + return __finalise_sg(dev, sg, nents, 0);
> +
> iova = iommu_dma_alloc_iova(domain, iova_len, dma_get_mask(dev), dev);
> if (!iova) {
> ret = -ENOMEM;
> @@ -1236,7 +1281,7 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> out_restore_sg:
> __invalidate_sg(sg, nents);
> out:
> - if (ret != -ENOMEM)
> + if (ret != -ENOMEM && ret != -EREMOTEIO)
> return -EINVAL;
> return ret;
> }
> @@ -1244,7 +1289,7 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> static void iommu_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
> int nents, enum dma_data_direction dir, unsigned long attrs)
> {
> - dma_addr_t start, end;
> + dma_addr_t end, start = DMA_MAPPING_ERROR;

There are several things I don't like about this logic, I'd rather have
"end = 0" here...

> struct scatterlist *tmp;
> int i;
>
> @@ -1260,14 +1305,22 @@ static void iommu_dma_unmap_sg(struct device *dev, struct scatterlist *sg,
> * The scatterlist segments are mapped into a single
> * contiguous IOVA allocation, so this is incredibly easy.
> */

[ This comment rather stops being true :( ]

> - start = sg_dma_address(sg);
> - for_each_sg(sg_next(sg), tmp, nents - 1, i) {

...then generalise the first-element special case here into a dedicated
"walk to the first non-p2p element" loop...

> + for_each_sg(sg, tmp, nents, i) {
> + if (sg_is_dma_bus_address(tmp)) {
> + sg_dma_unmark_bus_address(tmp);

[ Again I question what this actually achieves ]

> + continue;
> + }
> if (sg_dma_len(tmp) == 0)
> break;
> - sg = tmp;
> +
> + if (start == DMA_MAPPING_ERROR)
> + start = sg_dma_address(tmp);
> +
> + end = sg_dma_address(tmp) + sg_dma_len(tmp);
> }
> - end = sg_dma_address(sg) + sg_dma_len(sg);
> - __iommu_dma_unmap(dev, start, end - start);
> +
> + if (start != DMA_MAPPING_ERROR)

...then "if (end)" here.

Thanks,
Robin.

> + __iommu_dma_unmap(dev, start, end - start);
> }
>
> static dma_addr_t iommu_dma_map_resource(struct device *dev, phys_addr_t phys,
> @@ -1460,6 +1513,7 @@ static unsigned long iommu_dma_get_merge_boundary(struct device *dev)
> }
>
> static const struct dma_map_ops iommu_dma_ops = {
> + .flags = DMA_F_PCI_P2PDMA_SUPPORTED,
> .alloc = iommu_dma_alloc,
> .free = iommu_dma_free,
> .alloc_pages = dma_common_alloc_pages,

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