Message-ID: <496443c7-b9f6-4ebb-b409-785236c03780@arm.com>
Date: Wed, 17 Dec 2025 14:23:35 +0000
From: Steven Price <steven.price@....com>
To: Adrián Larumbe <adrian.larumbe@...labora.com>,
 linux-kernel@...r.kernel.org
Cc: dri-devel@...ts.freedesktop.org,
 Boris Brezillon <boris.brezillon@...labora.com>, kernel@...labora.com,
 Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Subject: Re: [PATCH v4 1/1] drm/panthor: Support partial unmaps of huge pages

On 14/12/2025 20:24, Adrián Larumbe wrote:
> Commit 33729a5fc0ca ("iommu/io-pgtable-arm: Remove split on unmap
> behavior") did away with the treatment of partial unmaps of huge IOPTEs.
> 
> In the case of Panthor, that means an attempt to run a VM_BIND unmap
> operation on a memory region whose start address and size aren't 2MiB
> aligned, in the event it intersects with a huge page, would lead to ARM
> IOMMU management code to fail and a warning being raised.
> 
> Presently, and for lack of a better alternative, it's best to have
> Panthor handle partial unmaps at the driver level, by unmapping entire
> huge pages and remapping the difference between them and the requested
> unmap region.
> 
> This could change in the future when the VM_BIND uAPI is expanded to
> enforce huge page alignment and map/unmap operational constraints that
> render this code unnecessary.
> 
> When a partial unmap for a huge PTE is attempted, we also need to expand
> the locked region to encompass whole huge pages.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@...labora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 99 ++++++++++++++++++++++++---
>  1 file changed, 91 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index a3e5e77fc9ed..cabee92e9580 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -547,12 +547,12 @@ static int as_send_cmd_and_wait(struct panthor_device *ptdev, u32 as_nr, u32 cmd
>  	return status;
>  }
>  
> -static u64 pack_region_range(struct panthor_device *ptdev, u64 region_start, u64 size)
> +static u64 pack_region_range(struct panthor_device *ptdev, u64 *region_start, u64 *size)
>  {
>  	u8 region_width;
> -	u64 region_end = region_start + size;
> +	u64 region_end = *region_start + *size;
>  
> -	if (drm_WARN_ON_ONCE(&ptdev->base, !size))
> +	if (drm_WARN_ON_ONCE(&ptdev->base, !*size))
>  		return 0;
>  
>  	/*
> @@ -563,16 +563,17 @@ static u64 pack_region_range(struct panthor_device *ptdev, u64 region_start, u64
>  	 * change, the desired region starts with this bit (and subsequent bits)
>  	 * zeroed and ends with the bit (and subsequent bits) set to one.
>  	 */
> -	region_width = max(fls64(region_start ^ (region_end - 1)),
> +	region_width = max(fls64(*region_start ^ (region_end - 1)),
>  			   const_ilog2(AS_LOCK_REGION_MIN_SIZE)) - 1;
>  
>  	/*
>  	 * Mask off the low bits of region_start (which would be ignored by
>  	 * the hardware anyway)
>  	 */
> -	region_start &= GENMASK_ULL(63, region_width);
> +	*region_start &= GENMASK_ULL(63, region_width);
> +	*size = 1ull << (region_width + 1);
>  
> -	return region_width | region_start;
> +	return region_width | *region_start;
>  }
>  
>  static int panthor_mmu_as_enable(struct panthor_device *ptdev, u32 as_nr,
> @@ -1691,12 +1692,19 @@ static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size)
>  	struct panthor_device *ptdev = vm->ptdev;
>  	int ret = 0;
>  
> +	/* sm_step_remap() can call panthor_vm_lock_region() to account for
> +	 * the wider unmap needed when doing a partial huge page unamp. We
> +	 * need to ignore the lock if it's already part of the locked region.
> +	 */
> +	if (start >= vm->locked_region.start &&
> +	    start + size <= vm->locked_region.start + vm->locked_region.size)
> +		return 0;
> +
>  	mutex_lock(&ptdev->mmu->as.slots_lock);
> -	drm_WARN_ON(&ptdev->base, vm->locked_region.start || vm->locked_region.size);
>  	if (vm->as.id >= 0 && size) {
>  		/* Lock the region that needs to be updated */
>  		gpu_write64(ptdev, AS_LOCKADDR(vm->as.id),
> -			    pack_region_range(ptdev, start, size));
> +			    pack_region_range(ptdev, &start, &size));
>  
>  		/* If the lock succeeded, update the locked_region info. */
>  		ret = as_send_cmd_and_wait(ptdev, vm->as.id, AS_COMMAND_LOCK);
> @@ -2169,6 +2177,48 @@ static int panthor_gpuva_sm_step_map(struct drm_gpuva_op *op, void *priv)
>  	return 0;
>  }
>  
> +static bool
> +iova_mapped_as_huge_page(struct drm_gpuva_op_map *op, u64 addr)
> +{
> +	const struct page *pg;
> +	pgoff_t bo_offset;
> +
> +	bo_offset = addr - op->va.addr + op->gem.offset;
> +	pg = to_panthor_bo(op->gem.obj)->base.pages[bo_offset >> PAGE_SHIFT];
> +
> +	return folio_size(page_folio(pg)) >= SZ_2M;
> +}
> +
> +static void
> +unmap_hugepage_align(const struct drm_gpuva_op_remap *op,
> +		     u64 *unmap_start, u64 *unmap_range)
> +{
> +	u64 aligned_unmap_start, aligned_unmap_end, unmap_end;
> +
> +	unmap_end = *unmap_start + *unmap_range;
> +	aligned_unmap_start = ALIGN_DOWN(*unmap_start, SZ_2M);
> +	aligned_unmap_end = ALIGN(unmap_end, SZ_2M);
> +
> +	/* If we're dealing with a huge page, make sure the unmap region is
> +	 * aligned on the start of the page.
> +	 */
> +	if (op->prev && aligned_unmap_start < *unmap_start &&
> +	    op->prev->va.addr <= aligned_unmap_start &&
> +	    iova_mapped_as_huge_page(op->prev, *unmap_start)) {
> +		*unmap_range += *unmap_start - aligned_unmap_start;
> +		*unmap_start = aligned_unmap_start;
> +	}
> +
> +	/* If we're dealing with a huge page, make sure the unmap region is
> +	 * aligned on the end of the page.
> +	 */
> +	if (op->next && aligned_unmap_end > unmap_end &&
> +	    op->next->va.addr + op->next->va.range >= aligned_unmap_end &&
> +	    iova_mapped_as_huge_page(op->next, unmap_end - 1)) {
> +		*unmap_range += aligned_unmap_end - unmap_end;
> +	}
> +}
> +
>  static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>  				       void *priv)
>  {
> @@ -2177,16 +2227,49 @@ static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>  	struct panthor_vm_op_ctx *op_ctx = vm->op_ctx;
>  	struct panthor_vma *prev_vma = NULL, *next_vma = NULL;
>  	u64 unmap_start, unmap_range;
> +	int ret;
>  
>  	drm_gpuva_op_remap_to_unmap_range(&op->remap, &unmap_start, &unmap_range);
> +	/*
> +	 * ARM IOMMU page table management code disallows partial unmaps of huge pages,
> +	 * so when a partial unmap is requested, we must first unmap the entire huge
> +	 * page and then remap the difference between the huge page minus the requested
> +	 * unmap region. Calculating the right start address and range for the expanded
> +	 * unmap operation is the responsibility of the following function.
> +	 */
> +	unmap_hugepage_align(&op->remap, &unmap_start, &unmap_range);
> +
> +	/* If the range changed, we might have to lock a wider region to guarantee
> +	 * atomicity. panthor_vm_lock_region() bails out early if the new region
> +	 * is already part of the locked region, so no need to do this check here.
> +	 */
> +	panthor_vm_lock_region(vm, unmap_start, unmap_start + unmap_range);

panthor_vm_lock_region() takes the start and *size*, but here you are
passing the start and end addresses. So this is going to lock more than
you intend.

Thanks,
Steve

>  	panthor_vm_unmap_pages(vm, unmap_start, unmap_range);
>  
>  	if (op->remap.prev) {
> +		struct panthor_gem_object *bo = to_panthor_bo(op->remap.prev->gem.obj);
> +		u64 offset = op->remap.prev->gem.offset + unmap_start - op->remap.prev->va.addr;
> +		u64 size = op->remap.prev->va.addr + op->remap.prev->va.range - unmap_start;
> +
> +		ret = panthor_vm_map_pages(vm, unmap_start, flags_to_prot(unmap_vma->flags),
> +					   bo->base.sgt, offset, size);
> +		if (ret)
> +			return ret;
> +
>  		prev_vma = panthor_vm_op_ctx_get_vma(op_ctx);
>  		panthor_vma_init(prev_vma, unmap_vma->flags);
>  	}
>  
>  	if (op->remap.next) {
> +		struct panthor_gem_object *bo = to_panthor_bo(op->remap.next->gem.obj);
> +		u64 addr = op->remap.next->va.addr;
> +		u64 size = unmap_start + unmap_range - op->remap.next->va.addr;
> +
> +		ret = panthor_vm_map_pages(vm, addr, flags_to_prot(unmap_vma->flags),
> +					   bo->base.sgt, op->remap.next->gem.offset, size);
> +		if (ret)
> +			return ret;
> +
>  		next_vma = panthor_vm_op_ctx_get_vma(op_ctx);
>  		panthor_vma_init(next_vma, unmap_vma->flags);
>  	}

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