Re: [PATCH v7 08/21] iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg
From: Robin Murphy
Date: Wed Jun 29 2022 - 08:13:17 EST
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@xxxxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
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,