Re: [RFC PATCH v3 02/11] drm/gpusvm: Use dma-map IOVA alloc, link, and sync API in GPU SVM

[Leon Romanovsky <leonro@nvidia.com>]

On Tue, Jan 27, 2026 at 04:48:32PM -0800, Matthew Brost wrote:
> The dma-map IOVA alloc, link, and sync APIs perform significantly better
> than dma-map / dma-unmap, as they avoid costly IOMMU synchronizations.
> This difference is especially noticeable when mapping a 2MB region in
> 4KB pages.
> 
> Use the IOVA alloc, link, and sync APIs for GPU SVM, which create DMA
> mappings between the CPU and GPU.
> 
> v3:
>  - Always link IOVA in mixed mappings
>  - Sync IOVA

The same comment as before.

> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>  drivers/gpu/drm/drm_gpusvm.c | 87 ++++++++++++++++++++++++++++--------
>  include/drm/drm_gpusvm.h     |  3 ++
>  2 files changed, 71 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gpusvm.c b/drivers/gpu/drm/drm_gpusvm.c
> index 585d913d3d19..084e78fa0f32 100644
> --- a/drivers/gpu/drm/drm_gpusvm.c
> +++ b/drivers/gpu/drm/drm_gpusvm.c
> @@ -1139,19 +1139,26 @@ static void __drm_gpusvm_unmap_pages(struct drm_gpusvm *gpusvm,
>  		struct drm_gpusvm_pages_flags flags = {
>  			.__flags = svm_pages->flags.__flags,
>  		};
> +		struct dma_iova_state __state = {};

Why don't you use svm_pages->state directly?

>  
> -		for (i = 0, j = 0; i < npages; j++) {
> -			struct drm_pagemap_addr *addr = &svm_pages->dma_addr[j];
> -
> -			if (addr->proto == DRM_INTERCONNECT_SYSTEM)
> -				dma_unmap_page(dev,
> -					       addr->addr,
> -					       PAGE_SIZE << addr->order,
> -					       addr->dir);
> -			else if (dpagemap && dpagemap->ops->device_unmap)
> -				dpagemap->ops->device_unmap(dpagemap,
> -							    dev, *addr);
> -			i += 1 << addr->order;
> +		if (dma_use_iova(&svm_pages->state)) {
> +			dma_iova_destroy(dev, &svm_pages->state,
> +					 npages * PAGE_SIZE,
> +					 svm_pages->dma_addr[0].dir, 0);
> +		} else {
> +			for (i = 0, j = 0; i < npages; j++) {
> +				struct drm_pagemap_addr *addr = &svm_pages->dma_addr[j];
> +
> +				if (addr->proto == DRM_INTERCONNECT_SYSTEM)
> +					dma_unmap_page(dev,
> +						       addr->addr,
> +						       PAGE_SIZE << addr->order,
> +						       addr->dir);
> +				else if (dpagemap && dpagemap->ops->device_unmap)
> +					dpagemap->ops->device_unmap(dpagemap,
> +								    dev, *addr);
> +				i += 1 << addr->order;
> +			}
>  		}
>  
>  		/* WRITE_ONCE pairs with READ_ONCE for opportunistic checks */
> @@ -1161,6 +1168,7 @@ static void __drm_gpusvm_unmap_pages(struct drm_gpusvm *gpusvm,
>  
>  		drm_pagemap_put(svm_pages->dpagemap);
>  		svm_pages->dpagemap = NULL;
> +		svm_pages->state = __state;
>  	}
>  }
>  
> @@ -1402,12 +1410,14 @@ int drm_gpusvm_get_pages(struct drm_gpusvm *gpusvm,
>  	unsigned long num_dma_mapped;
>  	unsigned int order = 0;
>  	unsigned long *pfns;
> +	phys_addr_t last_phys;
>  	int err = 0;
>  	struct dev_pagemap *pagemap;
>  	struct drm_pagemap *dpagemap;
>  	struct drm_gpusvm_pages_flags flags;
>  	enum dma_data_direction dma_dir = ctx->read_only ? DMA_TO_DEVICE :
>  							   DMA_BIDIRECTIONAL;
> +	struct dma_iova_state *state = &svm_pages->state;
>  
>  retry:
>  	if (time_after(jiffies, timeout))
> @@ -1496,6 +1506,17 @@ int drm_gpusvm_get_pages(struct drm_gpusvm *gpusvm,
>  				err = -EOPNOTSUPP;
>  				goto err_unmap;
>  			}
> +
> +			if (dma_use_iova(state)) {
> +				err = dma_iova_link(gpusvm->drm->dev, state,
> +						    last_phys,
> +						    i * PAGE_SIZE,
> +						    PAGE_SIZE << order,
> +						    dma_dir, 0);
> +				if (err)
> +					goto err_unmap;
> +			}
> +
>  			zdd = __zdd;
>  			if (pagemap != page_pgmap(page)) {
>  				if (i > 0) {
> @@ -1539,13 +1560,34 @@ int drm_gpusvm_get_pages(struct drm_gpusvm *gpusvm,
>  				goto err_unmap;
>  			}
>  
> -			addr = dma_map_page(gpusvm->drm->dev,
> -					    page, 0,
> -					    PAGE_SIZE << order,
> -					    dma_dir);
> -			if (dma_mapping_error(gpusvm->drm->dev, addr)) {
> -				err = -EFAULT;
> -				goto err_unmap;
> +			if (!i)
> +				dma_iova_try_alloc(gpusvm->drm->dev, state,
> +						   npages * PAGE_SIZE >=
> +						   HPAGE_PMD_SIZE ?
> +						   HPAGE_PMD_SIZE : 0,
> +						   npages * PAGE_SIZE);
> +
> +			if (dma_use_iova(state)) {
> +				last_phys = page_to_phys(page);

hmm_pfn_to_phys().

> +
> +				err = dma_iova_link(gpusvm->drm->dev, state,
> +						    page_to_phys(page),

you have last_phys here.

> +						    i * PAGE_SIZE,
> +						    PAGE_SIZE << order,
> +						    dma_dir, 0);
> +				if (err)
> +					goto err_unmap;
> +

Thanks