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 01:58:06PM +0800, Baolu Lu wrote:
> On 4/29/25 13:53, Leon Romanovsky wrote:
> > 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 }
> 
> 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)
> {
> <...>
> 	/*
> 	 * 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))
> 		return phys;
> <...>
> }
> 
> Then,
> 
> 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)
> {
> <...>
> 	phys = iommu_dma_map_swiotlb(dev, phys, size, dir, attrs);
> 	if (phys == (phys_addr_t)DMA_MAPPING_ERROR)
> 		return DMA_MAPPING_ERROR;
> <...>
> }

Such change will cause to extra function call for everyone who doesn't
use SWIOTLB (RDMA, HMM e.t.c).

In addition, iommu_dma_map_swiotlb() is called through
dma_iova_link -> 
	iommu_dma_iova_link_swiotlb -> 
		iommu_dma_iova_bounce_and_link() -> 
			iommu_dma_map_swiotlb()
and dma_iova_link() has this "if (dev_use_swiotlb(dev, size, dir) && iova_unaligned(iovad, phys, size))"
very early at call stack.

So, in dma_iova_link() we will find ourselves with same check twice.

Thanks

> 
> Thanks,
> baolu
>