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

[Baolu Lu <baolu.lu@linux.intel.com>]

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;
<...>
}

Thanks,
baolu