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

Thomas Hellström
2026-02-09T16:41:48Z

On Mon, 2026-02-09 at 08:13 -0800, Matthew Brost wrote:
> On Mon, Feb 09, 2026 at 10:44:43AM +0100, Thomas Hellström wrote:
> > 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?
> >
>
>  * Note: @phys is only used to calculate the IOVA alignment. Callers
> that always
>  * do PAGE_SIZE aligned transfers can safely pass 0 here.
>
> So 0 would be safe but possibly suboptimal. For mapping greater than
> or
> equal to 2M, we'd like 2M MB alignment so large GPU pages can used
> too.
> I think passing in '0' could result in odd alignment.
>
> I am assuming other vendors have 2M GPU pages here too but that seems
> like somewhat safe assumption...

Ah, I interpreted that as beyond PAGE_SIZE the function would behave
the same.

Agree that if we can get 2M alignment that's much better.

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

>
> Matt
>
> >
> > > +    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>