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Message-ID: <cddcaeff-9652-4685-ba6c-0fda79bd8680@arm.com>
Date: Thu, 8 Jan 2026 11:01:35 +0000
From: Robin Murphy <robin.murphy@....com>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>,
iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
linux-coco@...ts.linux.dev
Cc: Marek Szyprowski <m.szyprowski@...sung.com>, steven.price@....com,
Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH] dma-direct: swiotlb: Skip encryption toggles for swiotlb
allocations
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@...nel.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);
> }
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