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

[Leon Romanovsky <leon@kernel.org>]

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
>