Re: [PATCH v10 03/24] iommu: generalize the batched sync after map interface

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

On 4/28/25 17:22, Leon Romanovsky wrote:
> From: Christoph Hellwig<hch@lst.de>
> 
> For the upcoming IOVA-based DMA API we want to batch the
> ops->iotlb_sync_map() call after mapping multiple IOVAs from
> dma-iommu without having a scatterlist. Improve the API.
> 
> Add a wrapper for the map_sync as iommu_sync_map() so that callers
> don't need to poke into the methods directly.
> 
> Formalize __iommu_map() into iommu_map_nosync() which requires the
> caller to call iommu_sync_map() after all maps are completed.
> 
> Refactor the existing sanity checks from all the different layers
> into iommu_map_nosync().
> 
> Signed-off-by: Christoph Hellwig<hch@lst.de>
> Acked-by: Will Deacon<will@kernel.org>
> Tested-by: Jens Axboe<axboe@kernel.dk>
> Reviewed-by: Jason Gunthorpe<jgg@nvidia.com>
> Reviewed-by: Luis Chamberlain<mcgrof@kernel.org>
> Signed-off-by: Leon Romanovsky<leonro@nvidia.com>
> ---
>   drivers/iommu/iommu.c | 65 +++++++++++++++++++------------------------
>   include/linux/iommu.h |  4 +++
>   2 files changed, 33 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 4f91a740c15f..02960585b8d4 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2443,8 +2443,8 @@ static size_t iommu_pgsize(struct iommu_domain *domain, unsigned long iova,
>   	return pgsize;
>   }
>   
> -static int __iommu_map(struct iommu_domain *domain, unsigned long iova,
> -		       phys_addr_t paddr, size_t size, int prot, gfp_t gfp)
> +int iommu_map_nosync(struct iommu_domain *domain, unsigned long iova,
> +		phys_addr_t paddr, size_t size, int prot, gfp_t gfp)
>   {
>   	const struct iommu_domain_ops *ops = domain->ops;
>   	unsigned long orig_iova = iova;
> @@ -2453,12 +2453,19 @@ static int __iommu_map(struct iommu_domain *domain, unsigned long iova,
>   	phys_addr_t orig_paddr = paddr;
>   	int ret = 0;
>   
> +	might_sleep_if(gfpflags_allow_blocking(gfp));
> +
>   	if (unlikely(!(domain->type & __IOMMU_DOMAIN_PAGING)))
>   		return -EINVAL;
>   
>   	if (WARN_ON(!ops->map_pages || domain->pgsize_bitmap == 0UL))
>   		return -ENODEV;
>   
> +	/* Discourage passing strange GFP flags */
> +	if (WARN_ON_ONCE(gfp & (__GFP_COMP | __GFP_DMA | __GFP_DMA32 |
> +				__GFP_HIGHMEM)))
> +		return -EINVAL;
> +
>   	/* find out the minimum page size supported */
>   	min_pagesz = 1 << __ffs(domain->pgsize_bitmap);
>   
> @@ -2506,31 +2513,27 @@ static int __iommu_map(struct iommu_domain *domain, unsigned long iova,
>   	return ret;
>   }
>   
> -int iommu_map(struct iommu_domain *domain, unsigned long iova,
> -	      phys_addr_t paddr, size_t size, int prot, gfp_t gfp)
> +int iommu_sync_map(struct iommu_domain *domain, unsigned long iova, size_t size)
>   {
>   	const struct iommu_domain_ops *ops = domain->ops;
> -	int ret;
> -
> -	might_sleep_if(gfpflags_allow_blocking(gfp));
>   
> -	/* Discourage passing strange GFP flags */
> -	if (WARN_ON_ONCE(gfp & (__GFP_COMP | __GFP_DMA | __GFP_DMA32 |
> -				__GFP_HIGHMEM)))
> -		return -EINVAL;
> +	if (!ops->iotlb_sync_map)
> +		return 0;
> +	return ops->iotlb_sync_map(domain, iova, size);
> +}

I am wondering whether iommu_sync_map() needs a return value. The
purpose of this callback is just to sync the TLB cache after new
mappings are created, which should effectively be a no-fail operation.

The definition of iotlb_sync_map in struct iommu_domain_ops seems
unnecessary:

struct iommu_domain_ops {
...
         int (*iotlb_sync_map)(struct iommu_domain *domain, unsigned 
long iova,
                               size_t size);
...
};

Furthermore, currently no iommu driver implements this callback in a way
that returns a failure. We could clean up the iommu definition in a
subsequent patch series, but for this driver-facing interface, it's
better to get it right from the beginning.

Thanks,
baolu