Re: [PATCH v10 06/24] iommu/dma: Factor out a iommu_dma_map_swiotlb helper

Leon Romanovsky
2025-04-29T05:53:43Z

On Tue, Apr 29, 2025 at 12:58:18PM +0800, Baolu Lu wrote:
> On 4/28/25 17:22, Leon Romanovsky wrote:
> > From: Christoph Hellwig<hch@lst.de>
> >
> > Split the iommu logic from iommu_dma_map_page into a separate helper.
> > This not only keeps the code neatly separated, but will also allow for
> > reuse in another caller.
> >
> > Signed-off-by: Christoph Hellwig<hch@lst.de>
> > Tested-by: Jens Axboe<axboe@kernel.dk>
> > Reviewed-by: Luis Chamberlain<mcgrof@kernel.org>
> > Signed-off-by: Leon Romanovsky<leonro@nvidia.com>
>
> Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
>
> with a nit below ...
>
> > ---
> > drivers/iommu/dma-iommu.c | 73 ++++++++++++++++++++++-----------------
> > 1 file changed, 41 insertions(+), 32 deletions(-)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index d3211a8d755e..d7684024c439 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -1138,6 +1138,43 @@ void iommu_dma_sync_sg_for_device(struct device *dev, struct scatterlist *sgl,
> > arch_sync_dma_for_device(sg_phys(sg), sg->length, dir);
> > }
> > +static phys_addr_t iommu_dma_map_swiotlb(struct device *dev, phys_addr_t phys,
> > + size_t size, enum dma_data_direction dir, unsigned long attrs)
> > +{
> > + struct iommu_domain *domain = iommu_get_dma_domain(dev);
> > + struct iova_domain *iovad = &domain->iova_cookie->iovad;
> > +
> > + if (!is_swiotlb_active(dev)) {
> > + dev_warn_once(dev, "DMA bounce buffers are inactive, unable to map unaligned transaction.\n");
> > + return (phys_addr_t)DMA_MAPPING_ERROR;
> > + }
> > +
> > + trace_swiotlb_bounced(dev, phys, size);
> > +
> > + phys = swiotlb_tbl_map_single(dev, phys, size, iova_mask(iovad), dir,
> > + attrs);
> > +
> > + /*
> > + * Untrusted devices should not see padding areas with random leftover
> > + * kernel data, so zero the pre- and post-padding.
> > + * swiotlb_tbl_map_single() has initialized the bounce buffer proper to
> > + * the contents of the original memory buffer.
> > + */
> > + if (phys != (phys_addr_t)DMA_MAPPING_ERROR && dev_is_untrusted(dev)) {
> > + size_t start, virt = (size_t)phys_to_virt(phys);
> > +
> > + /* Pre-padding */
> > + start = iova_align_down(iovad, virt);
> > + memset((void *)start, 0, virt - start);
> > +
> > + /* Post-padding */
> > + start = virt + size;
> > + memset((void *)start, 0, iova_align(iovad, start) - start);
> > + }
> > +
> > + return phys;
> > +}
> > +
> > dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page,
> > unsigned long offset, size_t size, enum dma_data_direction dir,
> > unsigned long attrs)
> > @@ -1151,42 +1188,14 @@ dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page,
> > dma_addr_t iova, dma_mask = dma_get_mask(dev);
> > /*
> > - * If both the physical buffer start address and size are
> > - * page aligned, we don't need to use a bounce page.
> > + * If both the physical buffer start address and size are page aligned,
> > + * we don't need to use a bounce page.
> > */
> > if (dev_use_swiotlb(dev, size, dir) &&
> > iova_offset(iovad, phys | size)) {
> > - if (!is_swiotlb_active(dev)) {
>
> ... Is it better to move this check into the helper? Simply no-op if a
> bounce page is not needed:
>
> if (!dev_use_swiotlb(dev, size, dir) ||
> !iova_offset(iovad, phys | size))
> return phys;

Am I missing something? iommu_dma_map_page() has more code after this
check, so it is not correct to return immediately:

1189 dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page,
1190 unsigned long offset, size_t size, enum dma_data_direction dir,
1191 unsigned long attrs)
1192 {

<...>

1201 /*
1202 * If both the physical buffer start address and size are page aligned,
1203 * we don't need to use a bounce page.
1204 */
1205 if (dev_use_swiotlb(dev, size, dir) &&
1206 iova_unaligned(iovad, phys, size)) {
1207 phys = iommu_dma_map_swiotlb(dev, phys, size, dir, attrs);
1208 if (phys == (phys_addr_t)DMA_MAPPING_ERROR)
1209 return DMA_MAPPING_ERROR;
1210 }
1211
1212 if (!coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
1213 arch_sync_dma_for_device(phys, size, dir);
1214
1215 iova = __iommu_dma_map(dev, phys, size, prot, dma_mask);
1216 if (iova == DMA_MAPPING_ERROR)
1217 swiotlb_tbl_unmap_single(dev, phys, size, dir, attrs);
1218 return iova;
1219 }

>
> Thanks,
> baolu
>