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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Robin Murphy <robin.murphy@arm.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@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);
>>   }