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Message-ID: <yq5a7btr5wii.fsf@kernel.org>
Date: Fri, 09 Jan 2026 08:21:17 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Robin Murphy <robin.murphy@....com>, 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
Robin Murphy <robin.murphy@....com> writes:
> 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.
>
But rmem_swiotlb_device_init() is also marking the entire pool decrypted
set_memory_decrypted((unsigned long)phys_to_virt(rmem->base),
rmem->size >> PAGE_SHIFT);
-aneesh
>
> 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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