Re: [PATCH] dma-direct: swiotlb: Skip encryption toggles for swiotlb allocations

[Robin Murphy <robin.murphy@arm.com>]

On 2026-01-02 3:54 pm, Aneesh Kumar K.V (Arm) wrote:
> Swiotlb backing pages are already mapped decrypted via
> swiotlb_update_mem_attributes(), so dma-direct does not need to call
> set_memory_decrypted() during allocation or re-encrypt the memory on
> free.
> 
> Handle swiotlb-backed buffers explicitly: obtain the DMA address and
> zero the linear mapping for lowmem pages, and bypass the decrypt/encrypt
> transitions when allocating/freeing from the swiotlb pool (detected via
> swiotlb_find_pool()).

swiotlb_update_mem_attributes() only applies to the default SWIOTLB 
buffer, while the dma_direct_alloc_swiotlb() path is only for private 
restricted pools (because the whole point is that restricted DMA devices 
cannot use the regular allocator/default pools). There is no redundancy 
here AFAICS.

Thanks,
Robin.

> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>   kernel/dma/direct.c | 56 +++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 46 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index faf1e41afde8..c4ef4457bd74 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -104,15 +104,27 @@ static void __dma_direct_free_pages(struct device *dev, struct page *page,
>   	dma_free_contiguous(dev, page, size);
>   }
>   
> -static struct page *dma_direct_alloc_swiotlb(struct device *dev, size_t size)
> +static struct page *dma_direct_alloc_swiotlb(struct device *dev, size_t size,
> +					     dma_addr_t *dma_handle)
>   {
> -	struct page *page = swiotlb_alloc(dev, size);
> +	void *lm_addr;
> +	struct page *page;
> +
> +	page = swiotlb_alloc(dev, size);
> +	if (!page)
> +		return NULL;
>   
> -	if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
> +	if (!dma_coherent_ok(dev, page_to_phys(page), size)) {
>   		swiotlb_free(dev, page, size);
>   		return NULL;
>   	}
> +	/* If HighMem let caller take care of creating a mapping */
> +	if (PageHighMem(page))
> +		return page;
>   
> +	lm_addr = page_address(page);
> +	memset(lm_addr, 0, size);
> +	*dma_handle = phys_to_dma_direct(dev, page_to_phys(page));
>   	return page;
>   }
>   
> @@ -125,9 +137,6 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
>   
>   	WARN_ON_ONCE(!PAGE_ALIGNED(size));
>   
> -	if (is_swiotlb_for_alloc(dev))
> -		return dma_direct_alloc_swiotlb(dev, size);
> -
>   	gfp |= dma_direct_optimal_gfp_mask(dev, &phys_limit);
>   	page = dma_alloc_contiguous(dev, size, gfp);
>   	if (page) {
> @@ -204,6 +213,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   		dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs)
>   {
>   	bool remap = false, set_uncached = false;
> +	bool mark_mem_decrypt = true;
>   	bool allow_highmem = true;
>   	struct page *page;
>   	void *ret;
> @@ -251,6 +261,14 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	    dma_direct_use_pool(dev, gfp))
>   		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>   
> +	if (is_swiotlb_for_alloc(dev)) {
> +		page = dma_direct_alloc_swiotlb(dev, size, dma_handle);
> +		if (page) {
> +			mark_mem_decrypt = false;
> +			goto setup_page;
> +		}
> +		return NULL;
> +	}
>   
>   	if (force_dma_unencrypted(dev))
>   		/*
> @@ -266,6 +284,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	if (!page)
>   		return NULL;
>   
> +setup_page:
>   	/*
>   	 * dma_alloc_contiguous can return highmem pages depending on a
>   	 * combination the cma= arguments and per-arch setup.  These need to be
> @@ -295,7 +314,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   		ret = page_address(page);
>   	}
>   
> -	if (force_dma_unencrypted(dev)) {
> +	if (mark_mem_decrypt && force_dma_unencrypted(dev)) {
>   		void *lm_addr;
>   
>   		lm_addr = page_address(page);
> @@ -316,7 +335,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	return ret;
>   
>   out_encrypt_pages:
> -	if (dma_set_encrypted(dev, page_address(page), size))
> +	if (mark_mem_decrypt && dma_set_encrypted(dev, page_address(page), size))
>   		return NULL;
>   out_free_pages:
>   	__dma_direct_free_pages(dev, page, size);
> @@ -328,6 +347,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   void dma_direct_free(struct device *dev, size_t size,
>   		void *cpu_addr, dma_addr_t dma_addr, unsigned long attrs)
>   {
> +	bool mark_mem_encrypted = true;
>   	unsigned int page_order = get_order(size);
>   
>   	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> @@ -356,6 +376,9 @@ void dma_direct_free(struct device *dev, size_t size,
>   	    dma_free_from_pool(dev, cpu_addr, PAGE_ALIGN(size)))
>   		return;
>   
> +	if (swiotlb_find_pool(dev, dma_to_phys(dev, dma_addr)))
> +		mark_mem_encrypted = false;
> +
>   	if (is_vmalloc_addr(cpu_addr)) {
>   		vunmap(cpu_addr);
>   	} else {
> @@ -363,7 +386,7 @@ void dma_direct_free(struct device *dev, size_t size,
>   			arch_dma_clear_uncached(cpu_addr, size);
>   	}
>   
> -	if (force_dma_unencrypted(dev)) {
> +	if (mark_mem_encrypted && force_dma_unencrypted(dev)) {
>   		void *lm_addr;
>   
>   		lm_addr = phys_to_virt(dma_to_phys(dev, dma_addr));
> @@ -385,6 +408,15 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
>   	if (force_dma_unencrypted(dev) && dma_direct_use_pool(dev, gfp))
>   		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>   
> +	if (is_swiotlb_for_alloc(dev)) {
> +		page = dma_direct_alloc_swiotlb(dev, size, dma_handle);
> +		if (page && PageHighMem(page)) {
> +			swiotlb_free(dev, page, size);
> +			return NULL;
> +		}
> +		return page;
> +	}
> +
>   	page = __dma_direct_alloc_pages(dev, size, gfp, false);
>   	if (!page)
>   		return NULL;
> @@ -404,13 +436,17 @@ void dma_direct_free_pages(struct device *dev, size_t size,
>   		enum dma_data_direction dir)
>   {
>   	void *vaddr = page_address(page);
> +	bool mark_mem_encrypted = true;
>   
>   	/* If cpu_addr is not from an atomic pool, dma_free_from_pool() fails */
>   	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
>   	    dma_free_from_pool(dev, vaddr, size))
>   		return;
>   
> -	if (dma_set_encrypted(dev, vaddr, size))
> +	if (swiotlb_find_pool(dev, page_to_phys(page)))
> +		mark_mem_encrypted = false;
> +
> +	if (mark_mem_encrypted && dma_set_encrypted(dev, vaddr, size))
>   		return;
>   	__dma_direct_free_pages(dev, page, size);
>   }