Re: [PATCH v4 2/4] drm/gpusvm: Use dma-map IOVA alloc, link, and sync API in GPU SVM

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2026-02-04 at 20:19 -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.
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
> v3:
>  - Always link IOVA in mixed mappings
>  - Sync IOVA
> v4:
>  - Initialize IOVA state in get_pages
>  - Use pack IOVA linking (Jason)
>  - s/page_to_phys/hmm_pfn_to_phys (Leon)
> 
>  drivers/gpu/drm/drm_gpusvm.c | 55 ++++++++++++++++++++++++++++++----
> --
>  include/drm/drm_gpusvm.h     |  5 ++++
>  2 files changed, 52 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gpusvm.c
> b/drivers/gpu/drm/drm_gpusvm.c
> index 4b8130a4ce95..800caaf0a783 100644
> --- a/drivers/gpu/drm/drm_gpusvm.c
> +++ b/drivers/gpu/drm/drm_gpusvm.c
> @@ -1139,11 +1139,19 @@ static void __drm_gpusvm_unmap_pages(struct
> drm_gpusvm *gpusvm,
>  		struct drm_gpusvm_pages_flags flags = {
>  			.__flags = svm_pages->flags.__flags,
>  		};
> +		bool use_iova = dma_use_iova(&svm_pages->state);
> +
> +		if (use_iova) {
> +			dma_iova_unlink(dev, &svm_pages->state, 0,
> +					svm_pages->state_offset,
> +					svm_pages->dma_addr[0].dir,
> 0);
> +			dma_iova_free(dev, &svm_pages->state);
> +		}
>  
>  		for (i = 0, j = 0; i < npages; j++) {
>  			struct drm_pagemap_addr *addr = &svm_pages-
> >dma_addr[j];
>  
> -			if (addr->proto == DRM_INTERCONNECT_SYSTEM)
> +			if (!use_iova && addr->proto ==
> DRM_INTERCONNECT_SYSTEM)
>  				dma_unmap_page(dev,
>  					       addr->addr,
>  					       PAGE_SIZE << addr-
> >order,
> @@ -1408,6 +1416,7 @@ int drm_gpusvm_get_pages(struct drm_gpusvm
> *gpusvm,
>  	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))
> @@ -1446,6 +1455,9 @@ int drm_gpusvm_get_pages(struct drm_gpusvm
> *gpusvm,
>  	if (err)
>  		goto err_free;
>  
> +	*state = (struct dma_iova_state){};
> +	svm_pages->state_offset = 0;
> +
>  map_pages:
>  	/*
>  	 * Perform all dma mappings under the notifier lock to not
> @@ -1539,13 +1551,33 @@ 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,

Doc says "callers that always do PAGE_SIZE aligned transfers can always
pass 0 here", so can be simplified?


> +						   npages *
> PAGE_SIZE);
> +
> +			if (dma_use_iova(state)) {
> +				err = dma_iova_link(gpusvm->drm-
> >dev, state,
> +						   
> hmm_pfn_to_phys(pfns[i]),
> +						    svm_pages-
> >state_offset,
> +						    PAGE_SIZE <<
> order,
> +						    dma_dir, 0);
> +				if (err)
> +					goto err_unmap;
> +
> +				addr = state->addr + svm_pages-
> >state_offset;
> +				svm_pages->state_offset += PAGE_SIZE
> << order;
> +			} else {
> +				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;
> +				}
>  			}
>  
>  			svm_pages->dma_addr[j] =
> drm_pagemap_addr_encode
> @@ -1557,6 +1589,13 @@ int drm_gpusvm_get_pages(struct drm_gpusvm
> *gpusvm,
>  		flags.has_dma_mapping = true;
>  	}
>  
> +	if (dma_use_iova(state)) {
> +		err = dma_iova_sync(gpusvm->drm->dev, state, 0,
> +				    svm_pages->state_offset);
> +		if (err)
> +			goto err_unmap;
> +	}
> +
>  	if (pagemap) {
>  		flags.has_devmem_pages = true;
>  		drm_pagemap_get(dpagemap);
> diff --git a/include/drm/drm_gpusvm.h b/include/drm/drm_gpusvm.h
> index 2578ac92a8d4..cd94bb2ee6ee 100644
> --- a/include/drm/drm_gpusvm.h
> +++ b/include/drm/drm_gpusvm.h
> @@ -6,6 +6,7 @@
>  #ifndef __DRM_GPUSVM_H__
>  #define __DRM_GPUSVM_H__
>  
> +#include <linux/dma-mapping.h>
>  #include <linux/kref.h>
>  #include <linux/interval_tree.h>
>  #include <linux/mmu_notifier.h>
> @@ -136,6 +137,8 @@ struct drm_gpusvm_pages_flags {
>   * @dma_addr: Device address array
>   * @dpagemap: The struct drm_pagemap of the device pages we're dma-
> mapping.
>   *            Note this is assuming only one drm_pagemap per range
> is allowed.
> + * @state: DMA IOVA state for mapping.
> + * @state_offset: DMA IOVA offset for mapping.
>   * @notifier_seq: Notifier sequence number of the range's pages
>   * @flags: Flags for range
>   * @flags.migrate_devmem: Flag indicating whether the range can be
> migrated to device memory
> @@ -147,6 +150,8 @@ struct drm_gpusvm_pages_flags {
>  struct drm_gpusvm_pages {
>  	struct drm_pagemap_addr *dma_addr;
>  	struct drm_pagemap *dpagemap;
> +	struct dma_iova_state state;
> +	unsigned long state_offset;
>  	unsigned long notifier_seq;
>  	struct drm_gpusvm_pages_flags flags;
>  };

Otherwise LGTM. 
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>