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Message-ID: <560b2653-ad00-9cc3-0645-df7aff278321@nvidia.com>
Date: Sun, 2 May 2021 18:14:40 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Logan Gunthorpe <logang@...tatee.com>,
<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>
CC: Stephen Bates <sbates@...thlin.com>,
Christoph Hellwig <hch@....de>,
Dan Williams <dan.j.williams@...el.com>,
Jason Gunthorpe <jgg@...pe.ca>,
Christian König <christian.koenig@....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>
Subject: Re: [PATCH 11/16] iommu/dma: Support PCI P2PDMA pages in dma-iommu
map_sg
On 4/8/21 10:01 AM, 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.
>
> 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@...tatee.com>
> ---
> drivers/iommu/dma-iommu.c | 66 ++++++++++++++++++++++++++++++++++-----
> 1 file changed, 58 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index af765c813cc8..ef49635f9819 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>
> @@ -864,6 +865,16 @@ 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) {
Newbie question: I'm in the dark as to why the !s_iova_len check is there,
can you please enlighten me?
> + if (i > 0)
> + cur = sg_next(cur);
> +
> + pci_p2pdma_map_bus_segment(s, cur);
> + count++;
> + cur_len = 0;
> + continue;
> + }
> +
This is really an if/else condition. And arguably, it would be better
to split out two subroutines, and call one or the other depending on
the result of if is_pci_p2pdma_page(), instead of this "continue" approach.
> /*
> * Now fill in the real DMA data. If...
> * - there is a valid output segment to append to
> @@ -961,10 +972,12 @@ 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);
> - int i;
> + int i, ret = 0;
>
> if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
> iommu_deferred_attach(dev, domain))
> @@ -993,6 +1006,31 @@ 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,
> + attrs);
> + }
> +
> + switch (map_type) {
> + case PCI_P2PDMA_MAP_BUS_ADDR:
> + /*
> + * A zero length will be ignored by
> + * iommu_map_sg() and then can be detected
> + * in __finalise_sg() to actually map the
> + * bus address.
> + */
> + s->length = 0;
> + continue;
> + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> + 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:
> @@ -1015,6 +1053,9 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> prev = s;
> }
>
> + if (!iova_len)
> + return __finalise_sg(dev, sg, nents, 0);
> +
ohhh, we're really slicing up this function pretty severely, what with the
continue and the early out and several other control flow changes. I think
it would be better to spend some time factoring this function into two
cases, now that you're adding a second case for PCI P2PDMA. Roughly,
two subroutines would do it.
As it is, this leaves behind a routine that is extremely hard to mentally
verify as correct.
thanks,
--
John Hubbard
NVIDIA
> iova = iommu_dma_alloc_iova(domain, iova_len, dma_get_mask(dev), dev);
> if (!iova)
> goto out_restore_sg;
> @@ -1032,13 +1073,13 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> 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 +1095,22 @@ 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.
> */
> - 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)) {
> + sg_unmark_pci_p2pdma(tmp);
> + 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 +1303,7 @@ 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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