Subject: Re: [PATCH v4 04/13] dma: swiotlb: track pool encryption state and honor DMA_ATTR_CC_SHARED

From: Aneesh Kumar K.V <[email-masked]>
Date: Thu, 21 May 2026 22:50:51 +0530

Mostafa Saleh <[email-masked]> writes:

> On Tue, May 12, 2026 at 10:05 AM Aneesh Kumar K.V (Arm)
> <[email-masked]> wrote:
>> @@ -1411,6 +1436,16 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
>>         if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
>>                 pr_warn_once("Memory encryption is active and system is using DMA bounce buffers\n");
>>
>> +       /*
>> +        * if we are trying to swiotlb map a decrypted paddr or the paddr is encrypted
>> +        * but the device is forcing decryption, use decrypted io_tlb_mem
>> +        */
>> +       if ((attrs & DMA_ATTR_CC_SHARED) || force_dma_unencrypted(dev))
>
> I don't think swiotlb needs to know about force_dma_unencrypted(), the
> dma/direct caller should have all the information to pass the
> appropriate flags.
>
> Thanks.
> Mostafa
>
>> +               require_decrypted = true;
>> +
>> +       if (require_decrypted != mem->unencrypted)
>> +               return (phys_addr_t)DMA_MAPPING_ERROR;
>> +

Based on other email threads, this is now updated to

@@ -1372,9 +1417,19 @@ static unsigned long mem_used(struct io_tlb_mem *mem)
  *			any pre- or post-padding for alignment
  * @alloc_align_mask:	Required start and end alignment of the allocated buffer
  * @dir:		DMA direction
- * @attrs:		Optional DMA attributes for the map operation
+ * @attrs:		Optional DMA attributes for the map operation, updated
+ *			to match the selected SWIOTLB pool
  *
  * Find and allocate a suitable sequence of IO TLB slots for the request.
+ * The device's SWIOTLB pool must match the device's current DMA encryption
+ * requirements. If the device requires decrypted DMA, bouncing is done through
+ * an unencrypted pool and the mapping is marked shared. If the device can DMA
+ * to encrypted memory, bouncing is done through an encrypted pool even when the
+ * original DMA address was unencrypted. Enabling encrypted DMA for a device is
+ * therefore expected to update its default io_tlb_mem to an encrypted pool, so
+ * later bounce mappings for both encrypted and decrypted original memory use
+ * that encrypted pool.
+ *
  * The allocated space starts at an alignment specified by alloc_align_mask,
  * and the size of the allocated space is rounded up so that the total amount
  * of allocated space is a multiple of (alloc_align_mask + 1). If
@@ -1411,6 +1466,16 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
 		pr_warn_once("Memory encryption is active and system is using DMA bounce buffers\n");
 
+	/* swiotlb pool is incorrect for this device */
+	if (unlikely(mem->unencrypted != force_dma_unencrypted(dev)))
+		return (phys_addr_t)DMA_MAPPING_ERROR;
+
+	/* Force attrs to match the kind of memory in the pool */
+	if (mem->unencrypted)
+		*attrs |= DMA_ATTR_CC_SHARED;
+	else
+		*attrs &= ~DMA_ATTR_CC_SHARED;
+
 	/*
 	 * The default swiotlb memory pool is allocated with PAGE_SIZE
 	 * alignment. If a mapping is requested with larger alignment,
@@ -1608,8 +1673,11 @@ dma_addr_t swiotlb_map(struct device *dev, phys_addr_t paddr, size_t size,
 	if (swiotlb_addr == (phys_addr_t)DMA_MAPPING_ERROR)
 		return DMA_MAPPING_ERROR;
 
-	/* Ensure that the address returned is DMA'ble */
-	dma_addr = phys_to_dma_unencrypted(dev, swiotlb_addr);
+	if (attrs & DMA_ATTR_CC_SHARED)
+		dma_addr = phys_to_dma_unencrypted(dev, swiotlb_addr);
+	else
+		dma_addr = phys_to_dma_encrypted(dev, swiotlb_addr);
+
 	if (unlikely(!dma_capable(dev, dma_addr, size, true))) {
 		__swiotlb_tbl_unmap_single(dev, swiotlb_addr, size, dir,
 			attrs | DMA_ATTR_SKIP_CPU_SYNC,
@@ -1773,7 +1841,7 @@ static inline void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,