Re: [RFC PATCH v2 08/11] iommu/dma: Support PCI P2PDMA pages in dma-iommu map_sg

Robin Murphy
2021-03-12T15:53:12Z

On 2021-03-11 23:31, 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.

This misled me at first, but I see the implementation does actually
appear to accomodate the case of working ACS where P2P *would* still
need to be mapped at the IOMMU.

> Similar to dma-direct, the sg_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>
> ---
> drivers/iommu/dma-iommu.c | 63 ++++++++++++++++++++++++++++++++-------
> 1 file changed, 53 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index af765c813cc8..c0821e9051a9 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -20,6 +20,7 @@
> #include <linux/mm.h>
> #include <linux/mutex.h>
> #include <linux/pci.h>
> +#include <linux/pci-p2pdma.h>
> #include <linux/swiotlb.h>
> #include <linux/scatterlist.h>
> #include <linux/vmalloc.h>
> @@ -846,7 +847,7 @@ static void iommu_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
> * segment's start address to avoid concatenating across one.
> */
> static int __finalise_sg(struct device *dev, struct scatterlist *sg, int nents,
> - dma_addr_t dma_addr)
> + dma_addr_t dma_addr, unsigned long attrs)
> {
> struct scatterlist *s, *cur = sg;
> unsigned long seg_mask = dma_get_seg_boundary(dev);
> @@ -864,6 +865,20 @@ 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) {
> + if (i > 0)
> + cur = sg_next(cur);
> +
> + sg_dma_address(cur) = sg_phys(s) + s->offset -

Are you sure about that? ;)

> + pci_p2pdma_bus_offset(sg_page(s));

Can the bus offset make P2P addresses overlap with regions of mem space
that we might use for regular IOVA allocation? That would be very bad...

> + sg_dma_len(cur) = s->length;
> + sg_mark_pci_p2pdma(cur);
> +
> + count++;
> + cur_len = 0;
> + continue;
> + }
> +
> /*
> * Now fill in the real DMA data. If...
> * - there is a valid output segment to append to
> @@ -960,11 +975,12 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> struct iommu_dma_cookie *cookie = domain->iova_cookie;
> struct iova_domain *iovad = &cookie->iovad;
> struct scatterlist *s, *prev = NULL;
> + struct dev_pagemap *pgmap = NULL;
> int prot = dma_info_to_prot(dir, dev_is_dma_coherent(dev), attrs);
> dma_addr_t iova;
> size_t iova_len = 0;
> unsigned long mask = dma_get_seg_boundary(dev);
> - int i;
> + int i, map = -1, ret = 0;
>
> if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
> iommu_deferred_attach(dev, domain))
> @@ -993,6 +1009,23 @@ 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 = pci_p2pdma_dma_map_type(dev, pgmap);
> + }
> +
> + if (map < 0) {

It rather feels like it should be the job of whoever creates the list in
the first place not to put unusable pages in it, especially since the
p2pdma_map_type looks to be a fairly coarse-grained and static thing.
The DMA API isn't responsible for validating normal memory pages, so
what makes P2P special?

> + ret = -EREMOTEIO;
> + goto out_restore_sg;
> + }
> +
> + if (map) {
> + s->length = 0;

I'm not really thrilled about the idea of passing zero-length segments
to iommu_map_sg(). Yes, it happens to trick the concatenation logic in
the current implementation into doing what you want, but it feels fragile.

> + continue;
> + }
> + }
> +
> /*
> * Due to the alignment of our single IOVA allocation, we can
> * depend on these assumptions about the segment boundary mask:
> @@ -1015,6 +1048,9 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> prev = s;
> }
>
> + if (!iova_len)
> + return __finalise_sg(dev, sg, nents, 0, attrs);
> +
> iova = iommu_dma_alloc_iova(domain, iova_len, dma_get_mask(dev), dev);
> if (!iova)
> goto out_restore_sg;
> @@ -1026,19 +1062,19 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> if (iommu_map_sg_atomic(domain, iova, sg, nents, prot) < iova_len)
> goto out_free_iova;
>
> - return __finalise_sg(dev, sg, nents, iova);
> + return __finalise_sg(dev, sg, nents, iova, attrs);
>
> out_free_iova:
> iommu_dma_free_iova(cookie, iova, iova_len, NULL);
> out_restore_sg:
> __invalidate_sg(sg, nents);
> - return 0;
> + return ret;
> }
>
> 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;
> struct scatterlist *tmp;
> int i;
>
> @@ -1054,14 +1090,20 @@ 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.
> */
> - start = sg_dma_address(sg);
> - for_each_sg(sg_next(sg), tmp, nents - 1, i) {
> + for_each_sg(sg, tmp, nents, i) {
> + if (sg_is_pci_p2pdma(tmp))

Since the flag is associated with the DMA address which will no longer
be valid, shouldn't it be cleared? The circumstances in which leaving it
around could cause a problem are tenuous, but definitely possible.

Robin.

> + 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)
> + __iommu_dma_unmap(dev, start, end - start);
> }
>
> static dma_addr_t iommu_dma_map_resource(struct device *dev, phys_addr_t phys,
> @@ -1254,6 +1296,7 @@ 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,