Re: [PATCH v5 27/32] drm/xe: Add SVM VRAM migration

[Matthew Auld <matthew.auld@intel.com>]

On 13/02/2025 02:11, Matthew Brost wrote:
> Migration is implemented with range granularity, with VRAM backing being
> a VM private TTM BO (i.e., shares dma-resv with VM). The lifetime of the
> TTM BO is limited to when the SVM range is in VRAM (i.e., when a VRAM
> SVM range is migrated to SRAM, the TTM BO is destroyed).
> 
> The design choice for using TTM BO for VRAM backing store, as opposed to
> direct buddy allocation, is as follows:
> 
> - DRM buddy allocations are not at page granularity, offering no
>    advantage over a BO.
> - Unified eviction is required (SVM VRAM and TTM BOs need to be able to
>    evict each other).
> - For exhaustive eviction [1], SVM VRAM allocations will almost certainly
>    require a dma-resv.
> - Likely allocation size is 2M which makes of size of BO (872)
>    acceptable per allocation (872 / 2M == .0004158).
> 
> With this, using TTM BO for VRAM backing store seems to be an obvious
> choice as it allows leveraging of the TTM eviction code.
> 
> Current migration policy is migrate any SVM range greater than or equal
> to 64k once.
> 
> [1] https://patchwork.freedesktop.org/series/133643/
> 
> v2:
>   - Rebase on latest GPU SVM
>   - Retry page fault on get pages returning mixed allocation
>   - Use drm_gpusvm_devmem
> v3:
>   - Use new BO flags
>   - New range structure (Thomas)
>   - Hide migration behind Kconfig
>   - Kernel doc (Thomas)
>   - Use check_pages_threshold
> v4:
>   - Don't evict partial unmaps in garbage collector (Thomas)
>   - Use %pe to print errors (Thomas)
>   - Use %p to print pointers (Thomas)
> v5:
>   - Use range size helper (Thomas)
>   - Make BO external (Thomas)
>   - Set tile to NULL for BO creation (Thomas)
>   - Drop BO mirror flag (Thomas)
>   - Hold BO dma-resv lock across migration (Auld, Thomas)
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>   drivers/gpu/drm/xe/xe_svm.c | 111 ++++++++++++++++++++++++++++++++++--
>   drivers/gpu/drm/xe/xe_svm.h |   5 ++
>   2 files changed, 112 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
> index 0a78a838508c..2e1e0f31c1a8 100644
> --- a/drivers/gpu/drm/xe/xe_svm.c
> +++ b/drivers/gpu/drm/xe/xe_svm.c
> @@ -32,6 +32,11 @@ static unsigned long xe_svm_range_end(struct xe_svm_range *range)
>   	return drm_gpusvm_range_end(&range->base);
>   }
>   
> +static unsigned long xe_svm_range_size(struct xe_svm_range *range)
> +{
> +	return drm_gpusvm_range_size(&range->base);
> +}
> +
>   static void *xe_svm_devm_owner(struct xe_device *xe)
>   {
>   	return xe;
> @@ -512,7 +517,6 @@ static int xe_svm_populate_devmem_pfn(struct drm_gpusvm_devmem *devmem_allocatio
>   	return 0;
>   }
>   
> -__maybe_unused
>   static const struct drm_gpusvm_devmem_ops gpusvm_devmem_ops = {
>   	.devmem_release = xe_svm_devmem_release,
>   	.populate_devmem_pfn = xe_svm_populate_devmem_pfn,
> @@ -592,6 +596,71 @@ static bool xe_svm_range_is_valid(struct xe_svm_range *range,
>   	return (range->tile_present & ~range->tile_invalidated) & BIT(tile->id);
>   }
>   
> +static struct xe_vram_region *tile_to_vr(struct xe_tile *tile)
> +{
> +	return &tile->mem.vram;
> +}
> +
> +static struct xe_bo *xe_svm_alloc_vram(struct xe_vm *vm, struct xe_tile *tile,
> +				       struct xe_svm_range *range,
> +				       const struct drm_gpusvm_ctx *ctx)
> +{
> +	struct mm_struct *mm = vm->svm.gpusvm.mm;
> +	struct xe_vram_region *vr = tile_to_vr(tile);
> +	struct drm_buddy_block *block;
> +	struct list_head *blocks;
> +	struct xe_bo *bo;
> +	ktime_t end = 0;
> +	int err;
> +
> +	if (!mmget_not_zero(mm))
> +		return ERR_PTR(-EFAULT);
> +	mmap_read_lock(mm);
> +
> +retry:
> +	bo = xe_bo_create_locked(tile_to_xe(tile), NULL, NULL,
> +				 xe_svm_range_size(range),
> +				 ttm_bo_type_device,
> +				 XE_BO_FLAG_VRAM_IF_DGFX(tile));

Just to confirm, there is nothing scary with the vram still potentially 
being used by the GPU at this point (like with an async eviction + clear 
op), right? At some point we have some kind of synchronisation before 
the user can touch this memory?

> +	if (IS_ERR(bo)) {
> +		err = PTR_ERR(bo);
> +		if (xe_vm_validate_should_retry(NULL, err, &end))
> +			goto retry;
> +		goto unlock;
> +	}
> +
> +	drm_gpusvm_devmem_init(&bo->devmem_allocation,
> +			       vm->xe->drm.dev, mm,
> +			       &gpusvm_devmem_ops,
> +			       &tile->mem.vram.dpagemap,
> +			       xe_svm_range_size(range));
> +
> +	blocks = &to_xe_ttm_vram_mgr_resource(bo->ttm.resource)->blocks;
> +	list_for_each_entry(block, blocks, link)
> +		block->private = vr;
> +
> +	/*
> +	 * Take ref because as soon as drm_gpusvm_migrate_to_devmem succeeds the
> +	 * creation ref can be dropped upon CPU fault or unmap.
> +	 */
> +	xe_bo_get(bo);
> +
> +	err = drm_gpusvm_migrate_to_devmem(&vm->svm.gpusvm, &range->base,
> +					   &bo->devmem_allocation, ctx);
> +	xe_bo_unlock(bo);
> +	if (err) {
> +		xe_bo_put(bo);	/* Local ref */
> +		xe_bo_put(bo);	/* Creation ref */
> +		bo = ERR_PTR(err);
> +	}
> +
> +unlock:
> +	mmap_read_unlock(mm);
> +	mmput(mm);
> +
> +	return bo;
> +}
> +
>   /**
>    * xe_svm_handle_pagefault() - SVM handle page fault
>    * @vm: The VM.
> @@ -600,7 +669,8 @@ static bool xe_svm_range_is_valid(struct xe_svm_range *range,
>    * @fault_addr: The GPU fault address.
>    * @atomic: The fault atomic access bit.
>    *
> - * Create GPU bindings for a SVM page fault.
> + * Create GPU bindings for a SVM page fault. Optionally migrate to device
> + * memory.
>    *
>    * Return: 0 on success, negative error code on error.
>    */
> @@ -608,11 +678,18 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>   			    struct xe_tile *tile, u64 fault_addr,
>   			    bool atomic)
>   {
> -	struct drm_gpusvm_ctx ctx = { .read_only = xe_vma_read_only(vma), };
> +	struct drm_gpusvm_ctx ctx = {
> +		.read_only = xe_vma_read_only(vma),
> +		.devmem_possible = IS_DGFX(vm->xe) &&
> +			IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR),
> +		.check_pages_threshold = IS_DGFX(vm->xe) &&
> +			IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR) ? SZ_64K : 0,
> +	};
>   	struct xe_svm_range *range;
>   	struct drm_gpusvm_range *r;
>   	struct drm_exec exec;
>   	struct dma_fence *fence;
> +	struct xe_bo *bo = NULL;
>   	ktime_t end = 0;
>   	int err;
>   
> @@ -620,6 +697,9 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>   	xe_assert(vm->xe, xe_vma_is_cpu_addr_mirror(vma));
>   
>   retry:
> +	xe_bo_put(bo);
> +	bo = NULL;
> +
>   	/* Always process UNMAPs first so view SVM ranges is current */
>   	err = xe_svm_garbage_collector(vm);
>   	if (err)
> @@ -635,9 +715,31 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>   	if (xe_svm_range_is_valid(range, tile))
>   		return 0;
>   
> +	/* XXX: Add migration policy, for now migrate range once */
> +	if (!range->migrated && range->base.flags.migrate_devmem &&
> +	    xe_svm_range_size(range) >= SZ_64K) {
> +		range->migrated = true;
> +
> +		bo = xe_svm_alloc_vram(vm, tile, range, &ctx);
> +		if (IS_ERR(bo)) {
> +			drm_info(&vm->xe->drm,
> +				 "VRAM allocation failed, falling back to retrying, asid=%u, errno %pe\n",
> +				 vm->usm.asid, bo);
> +			bo = NULL;
> +			goto retry;
> +		}
> +	}
> +
>   	err = drm_gpusvm_range_get_pages(&vm->svm.gpusvm, r, &ctx);
> -	if (err == -EFAULT || err == -EPERM)	/* Corner where CPU mappings have changed */
> +	/* Corner where CPU mappings have changed */
> +	if (err == -EOPNOTSUPP || err == -EFAULT || err == -EPERM) {
> +		if (err == -EOPNOTSUPP)
> +			drm_gpusvm_range_evict(&vm->svm.gpusvm, &range->base);
> +		drm_info(&vm->xe->drm,
> +			 "Get pages failed, falling back to retrying, asid=%u, gpusvm=%p, errno %pe\n",
> +			 vm->usm.asid, &vm->svm.gpusvm, ERR_PTR(err));
>   		goto retry;
> +	}
>   	if (err)
>   		goto err_out;
>   
> @@ -668,6 +770,7 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>   	dma_fence_put(fence);
>   
>   err_out:
> +	xe_bo_put(bo);
>   
>   	return err;
>   }
> diff --git a/drivers/gpu/drm/xe/xe_svm.h b/drivers/gpu/drm/xe/xe_svm.h
> index 0fa525d34987..984a61651d9e 100644
> --- a/drivers/gpu/drm/xe/xe_svm.h
> +++ b/drivers/gpu/drm/xe/xe_svm.h
> @@ -35,6 +35,11 @@ struct xe_svm_range {
>   	 * range. Protected by GPU SVM notifier lock.
>   	 */
>   	u8 tile_invalidated;
> +	/**
> +	 * @migrated: Range has been migrated to device memory, protected by
> +	 * GPU fault handler locking.
> +	 */
> +	u8 migrated	:1;
>   };
>   
>   int xe_devm_add(struct xe_tile *tile, struct xe_vram_region *vr);