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

Baolu Lu
2025-04-29T06:02:26Z

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