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

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

Mostafa Saleh <smostafa@google.com> writes:

> On Thu, May 14, 2026 at 11:24:42AM +0530, Aneesh Kumar K.V wrote:
>> Mostafa Saleh <smostafa@google.com> writes:
>> 
>> > On Tue, May 12, 2026 at 02:33:59PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> >> Teach swiotlb to distinguish between encrypted and decrypted bounce
>> >> buffer pools, and make allocation and mapping paths select a pool whose
>> >> state matches the requested DMA attributes.
>> >> 
>> >> Add a decrypted flag to io_tlb_mem, initialize it for the default and
>> >> restricted pools, and propagate DMA_ATTR_CC_SHARED into swiotlb pool
>> >> allocation. Reject swiotlb alloc/map requests when the selected pool does
>> >> not match the required encrypted/decrypted state.
>> >> 
>> >> Also return DMA addresses with the matching phys_to_dma_{encrypted,
>> >> unencrypted} helper so the DMA address encoding stays consistent with the
>> >> chosen pool.
>> >> 
>> >> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> >> ---
>> >>  include/linux/dma-direct.h |  10 ++++
>> >>  include/linux/swiotlb.h    |   8 ++-
>> >>  kernel/dma/direct.c        |  14 +++--
>> >>  kernel/dma/swiotlb.c       | 108 +++++++++++++++++++++++++++----------
>> >>  4 files changed, 107 insertions(+), 33 deletions(-)
>> >> 
>> >> diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
>> >> index c249912456f9..94fad4e7c11e 100644
>> >> --- a/include/linux/dma-direct.h
>> >> +++ b/include/linux/dma-direct.h
>> >> @@ -77,6 +77,10 @@ static inline dma_addr_t dma_range_map_max(const struct bus_dma_region *map)
>> >>  #ifndef phys_to_dma_unencrypted
>> >>  #define phys_to_dma_unencrypted		phys_to_dma
>> >>  #endif
>> >> +
>> >> +#ifndef phys_to_dma_encrypted
>> >> +#define phys_to_dma_encrypted		phys_to_dma
>> >> +#endif
>> >>  #else
>> >>  static inline dma_addr_t __phys_to_dma(struct device *dev, phys_addr_t paddr)
>> >>  {
>> >> @@ -90,6 +94,12 @@ static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
>> >>  {
>> >>  	return dma_addr_unencrypted(__phys_to_dma(dev, paddr));
>> >>  }
>> >> +
>> >> +static inline dma_addr_t phys_to_dma_encrypted(struct device *dev,
>> >> +		phys_addr_t paddr)
>> >> +{
>> >> +	return dma_addr_encrypted(__phys_to_dma(dev, paddr));
>> >> +}
>> >>  /*
>> >>   * If memory encryption is supported, phys_to_dma will set the memory encryption
>> >>   * bit in the DMA address, and dma_to_phys will clear it.
>> >> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
>> >> index 3dae0f592063..b3fa3c6e0169 100644
>> >> --- a/include/linux/swiotlb.h
>> >> +++ b/include/linux/swiotlb.h
>> >> @@ -81,6 +81,7 @@ struct io_tlb_pool {
>> >>  	struct list_head node;
>> >>  	struct rcu_head rcu;
>> >>  	bool transient;
>> >> +	bool unencrypted;
>> >>  #endif
>> >>  };
>> >>  
>> >> @@ -111,6 +112,7 @@ struct io_tlb_mem {
>> >>  	struct dentry *debugfs;
>> >>  	bool force_bounce;
>> >>  	bool for_alloc;
>> >> +	bool unencrypted;
>> >>  #ifdef CONFIG_SWIOTLB_DYNAMIC
>> >>  	bool can_grow;
>> >>  	u64 phys_limit;
>> >> @@ -282,7 +284,8 @@ static inline void swiotlb_sync_single_for_cpu(struct device *dev,
>> >>  extern void swiotlb_print_info(void);
>> >>  
>> >>  #ifdef CONFIG_DMA_RESTRICTED_POOL
>> >> -struct page *swiotlb_alloc(struct device *dev, size_t size);
>> >> +struct page *swiotlb_alloc(struct device *dev, size_t size,
>> >> +		unsigned long attrs);
>> >>  bool swiotlb_free(struct device *dev, struct page *page, size_t size);
>> >>  
>> >>  static inline bool is_swiotlb_for_alloc(struct device *dev)
>> >> @@ -290,7 +293,8 @@ static inline bool is_swiotlb_for_alloc(struct device *dev)
>> >>  	return dev->dma_io_tlb_mem->for_alloc;
>> >>  }
>> >>  #else
>> >> -static inline struct page *swiotlb_alloc(struct device *dev, size_t size)
>> >> +static inline struct page *swiotlb_alloc(struct device *dev, size_t size,
>> >> +		unsigned long attrs)
>> >>  {
>> >>  	return NULL;
>> >>  }
>> >> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
>> >> index dc2907439b3d..97ae4fa10521 100644
>> >> --- a/kernel/dma/direct.c
>> >> +++ b/kernel/dma/direct.c
>> >> @@ -104,9 +104,10 @@ 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,
>> >> +		unsigned long attrs)
>> >>  {
>> >> -	struct page *page = swiotlb_alloc(dev, size);
>> >> +	struct page *page = swiotlb_alloc(dev, size, attrs);
>> >>  
>> >>  	if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
>> >>  		swiotlb_free(dev, page, size);
>> >> @@ -266,8 +267,12 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>> >>  						  gfp, attrs);
>> >>  
>> >>  	if (is_swiotlb_for_alloc(dev)) {
>> >> -		page = dma_direct_alloc_swiotlb(dev, size);
>> >> +		page = dma_direct_alloc_swiotlb(dev, size, attrs);
>> >>  		if (page) {
>> >> +			/*
>> >> +			 * swiotlb allocations comes from pool already marked
>> >> +			 * decrypted
>> >> +			 */
>> >>  			mark_mem_decrypt = false;
>> >>  			goto setup_page;
>> >>  		}
>> >> @@ -374,6 +379,7 @@ void dma_direct_free(struct device *dev, size_t size,
>> >>  		return;
>> >>  
>> >>  	if (swiotlb_find_pool(dev, dma_to_phys(dev, dma_addr)))
>> >> +		/* Swiotlb doesn't need a page attribute update on free */
>> >>  		mark_mem_encrypted = false;
>> >>  
>> >>  	if (is_vmalloc_addr(cpu_addr)) {
>> >> @@ -403,7 +409,7 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
>> >>  						  gfp, attrs);
>> >>  
>> >>  	if (is_swiotlb_for_alloc(dev)) {
>> >> -		page = dma_direct_alloc_swiotlb(dev, size);
>> >> +		page = dma_direct_alloc_swiotlb(dev, size, attrs);
>> >>  		if (!page)
>> >>  			return NULL;
>> >>  
>> >> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>> >> index ab4eccbaa076..065663be282c 100644
>> >> --- a/kernel/dma/swiotlb.c
>> >> +++ b/kernel/dma/swiotlb.c
>> >> @@ -259,10 +259,21 @@ void __init swiotlb_update_mem_attributes(void)
>> >>  	struct io_tlb_pool *mem = &io_tlb_default_mem.defpool;
>> >>  	unsigned long bytes;
>> >>  
>> >> +	/*
>> >> +	 * if platform support memory encryption, swiotlb buffers are
>> >> +	 * decrypted by default.
>> >> +	 */
>> >> +	if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
>> >> +		io_tlb_default_mem.unencrypted = true;
>> >> +	else
>> >> +		io_tlb_default_mem.unencrypted = false;
>> >> +
>> >>  	if (!mem->nslabs || mem->late_alloc)
>> >>  		return;
>> >>  	bytes = PAGE_ALIGN(mem->nslabs << IO_TLB_SHIFT);
>> >> -	set_memory_decrypted((unsigned long)mem->vaddr, bytes >> PAGE_SHIFT);
>> >> +
>> >> +	if (io_tlb_default_mem.unencrypted)
>> >> +		set_memory_decrypted((unsigned long)mem->vaddr, bytes >> PAGE_SHIFT);
>> >>  }
>> >>  
>> >>  static void swiotlb_init_io_tlb_pool(struct io_tlb_pool *mem, phys_addr_t start,
>> >> @@ -505,8 +516,10 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
>> >>  	if (!mem->slots)
>> >>  		goto error_slots;
>> >>  
>> >> -	set_memory_decrypted((unsigned long)vstart,
>> >> -			     (nslabs << IO_TLB_SHIFT) >> PAGE_SHIFT);
>> >> +	if (io_tlb_default_mem.unencrypted)
>> >> +		set_memory_decrypted((unsigned long)vstart,
>> >> +				     (nslabs << IO_TLB_SHIFT) >> PAGE_SHIFT);
>> >> +
>> >>  	swiotlb_init_io_tlb_pool(mem, virt_to_phys(vstart), nslabs, true,
>> >>  				 nareas);
>> >>  	add_mem_pool(&io_tlb_default_mem, mem);
>> >> @@ -539,7 +552,9 @@ void __init swiotlb_exit(void)
>> >>  	tbl_size = PAGE_ALIGN(mem->end - mem->start);
>> >>  	slots_size = PAGE_ALIGN(array_size(sizeof(*mem->slots), mem->nslabs));
>> >>  
>> >> -	set_memory_encrypted(tbl_vaddr, tbl_size >> PAGE_SHIFT);
>> >> +	if (io_tlb_default_mem.unencrypted)
>> >> +		set_memory_encrypted(tbl_vaddr, tbl_size >> PAGE_SHIFT);
>> >> +
>> >>  	if (mem->late_alloc) {
>> >>  		area_order = get_order(array_size(sizeof(*mem->areas),
>> >>  			mem->nareas));
>> >> @@ -563,6 +578,7 @@ void __init swiotlb_exit(void)
>> >>   * @gfp:	GFP flags for the allocation.
>> >>   * @bytes:	Size of the buffer.
>> >>   * @phys_limit:	Maximum allowed physical address of the buffer.
>> >> + * @unencrypted: true to allocate unencrypted memory, false for encrypted memory
>> >>   *
>> >>   * Allocate pages from the buddy allocator. If successful, make the allocated
>> >>   * pages decrypted that they can be used for DMA.
>> >> @@ -570,7 +586,8 @@ void __init swiotlb_exit(void)
>> >>   * Return: Decrypted pages, %NULL on allocation failure, or ERR_PTR(-EAGAIN)
>> >>   * if the allocated physical address was above @phys_limit.
>> >>   */
>> >> -static struct page *alloc_dma_pages(gfp_t gfp, size_t bytes, u64 phys_limit)
>> >> +static struct page *alloc_dma_pages(gfp_t gfp, size_t bytes,
>> >> +		u64 phys_limit, bool unencrypted)
>> >>  {
>> >>  	unsigned int order = get_order(bytes);
>> >>  	struct page *page;
>> >> @@ -588,13 +605,13 @@ static struct page *alloc_dma_pages(gfp_t gfp, size_t bytes, u64 phys_limit)
>> >>  	}
>> >>  
>> >>  	vaddr = phys_to_virt(paddr);
>> >> -	if (set_memory_decrypted((unsigned long)vaddr, PFN_UP(bytes)))
>> >> +	if (unencrypted && set_memory_decrypted((unsigned long)vaddr, PFN_UP(bytes)))
>> >>  		goto error;
>> >>  	return page;
>> >>  
>> >>  error:
>> >>  	/* Intentional leak if pages cannot be encrypted again. */
>> >> -	if (!set_memory_encrypted((unsigned long)vaddr, PFN_UP(bytes)))
>> >> +	if (unencrypted && !set_memory_encrypted((unsigned long)vaddr, PFN_UP(bytes)))
>> >>  		__free_pages(page, order);
>> >>  	return NULL;
>> >>  }
>> >> @@ -604,30 +621,26 @@ static struct page *alloc_dma_pages(gfp_t gfp, size_t bytes, u64 phys_limit)
>> >>   * @dev:	Device for which a memory pool is allocated.
>> >>   * @bytes:	Size of the buffer.
>> >>   * @phys_limit:	Maximum allowed physical address of the buffer.
>> >> + * @attrs:	DMA attributes for the allocation.
>> >>   * @gfp:	GFP flags for the allocation.
>> >>   *
>> >>   * Return: Allocated pages, or %NULL on allocation failure.
>> >>   */
>> >>  static struct page *swiotlb_alloc_tlb(struct device *dev, size_t bytes,
>> >> -		u64 phys_limit, gfp_t gfp)
>> >> +		u64 phys_limit, unsigned long attrs, gfp_t gfp)
>> >>  {
>> >>  	struct page *page;
>> >> -	unsigned long attrs = 0;
>> >>  
>> >>  	/*
>> >>  	 * Allocate from the atomic pools if memory is encrypted and
>> >>  	 * the allocation is atomic, because decrypting may block.
>> >>  	 */
>> >> -	if (!gfpflags_allow_blocking(gfp) && dev && force_dma_unencrypted(dev)) {
>> >> +	if (!gfpflags_allow_blocking(gfp) && (attrs & DMA_ATTR_CC_SHARED)) {
>> >>  		void *vaddr;
>> >>  
>> >>  		if (!IS_ENABLED(CONFIG_DMA_COHERENT_POOL))
>> >>  			return NULL;
>> >>  
>> >> -		/* swiotlb considered decrypted by default */
>> >> -		if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
>> >> -			attrs = DMA_ATTR_CC_SHARED;
>> >> -
>> >>  		return dma_alloc_from_pool(dev, bytes, &vaddr, gfp,
>> >>  					   attrs, dma_coherent_ok);
>> >>  	}
>> >> @@ -638,7 +651,8 @@ static struct page *swiotlb_alloc_tlb(struct device *dev, size_t bytes,
>> >>  	else if (phys_limit <= DMA_BIT_MASK(32))
>> >>  		gfp |= __GFP_DMA32;
>> >>  
>> >> -	while (IS_ERR(page = alloc_dma_pages(gfp, bytes, phys_limit))) {
>> >> +	while (IS_ERR(page = alloc_dma_pages(gfp, bytes, phys_limit,
>> >> +					     !!(attrs & DMA_ATTR_CC_SHARED)))) {
>> >>  		if (IS_ENABLED(CONFIG_ZONE_DMA32) &&
>> >>  		    phys_limit < DMA_BIT_MASK(64) &&
>> >>  		    !(gfp & (__GFP_DMA32 | __GFP_DMA)))
>> >> @@ -657,15 +671,18 @@ static struct page *swiotlb_alloc_tlb(struct device *dev, size_t bytes,
>> >>   * swiotlb_free_tlb() - free a dynamically allocated IO TLB buffer
>> >>   * @vaddr:	Virtual address of the buffer.
>> >>   * @bytes:	Size of the buffer.
>> >> + * @unencrypted: true if @vaddr was allocated decrypted and must be
>> >> + *	re-encrypted before being freed
>> >>   */
>> >> -static void swiotlb_free_tlb(void *vaddr, size_t bytes)
>> >> +static void swiotlb_free_tlb(void *vaddr, size_t bytes, bool unencrypted)
>> >>  {
>> >>  	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
>> >>  	    dma_free_from_pool(NULL, vaddr, bytes))
>> >>  		return;
>> >>  
>> >>  	/* Intentional leak if pages cannot be encrypted again. */
>> >> -	if (!set_memory_encrypted((unsigned long)vaddr, PFN_UP(bytes)))
>> >> +	if (!unencrypted ||
>> >> +	    !set_memory_encrypted((unsigned long)vaddr, PFN_UP(bytes)))
>> >>  		__free_pages(virt_to_page(vaddr), get_order(bytes));
>> >>  }
>> >>  
>> >> @@ -676,6 +693,7 @@ static void swiotlb_free_tlb(void *vaddr, size_t bytes)
>> >>   * @nslabs:	Desired (maximum) number of slabs.
>> >>   * @nareas:	Number of areas.
>> >>   * @phys_limit:	Maximum DMA buffer physical address.
>> >> + * @attrs:	DMA attributes for the allocation.
>> >>   * @gfp:	GFP flags for the allocations.
>> >>   *
>> >>   * Allocate and initialize a new IO TLB memory pool. The actual number of
>> >> @@ -686,7 +704,8 @@ static void swiotlb_free_tlb(void *vaddr, size_t bytes)
>> >>   */
>> >>  static struct io_tlb_pool *swiotlb_alloc_pool(struct device *dev,
>> >>  		unsigned long minslabs, unsigned long nslabs,
>> >> -		unsigned int nareas, u64 phys_limit, gfp_t gfp)
>> >> +		unsigned int nareas, u64 phys_limit, unsigned long attrs,
>> >> +		gfp_t gfp)
>> >>  {
>> >>  	struct io_tlb_pool *pool;
>> >>  	unsigned int slot_order;
>> >> @@ -704,9 +723,10 @@ static struct io_tlb_pool *swiotlb_alloc_pool(struct device *dev,
>> >>  	if (!pool)
>> >>  		goto error;
>> >>  	pool->areas = (void *)pool + sizeof(*pool);
>> >> +	pool->unencrypted = !!(attrs & DMA_ATTR_CC_SHARED);
>> >>  
>> >>  	tlb_size = nslabs << IO_TLB_SHIFT;
>> >> -	while (!(tlb = swiotlb_alloc_tlb(dev, tlb_size, phys_limit, gfp))) {
>> >> +	while (!(tlb = swiotlb_alloc_tlb(dev, tlb_size, phys_limit, attrs, gfp))) {
>> >>  		if (nslabs <= minslabs)
>> >>  			goto error_tlb;
>> >>  		nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE);
>> >> @@ -724,7 +744,8 @@ static struct io_tlb_pool *swiotlb_alloc_pool(struct device *dev,
>> >>  	return pool;
>> >>  
>> >>  error_slots:
>> >> -	swiotlb_free_tlb(page_address(tlb), tlb_size);
>> >> +	swiotlb_free_tlb(page_address(tlb), tlb_size,
>> >> +			 !!(attrs & DMA_ATTR_CC_SHARED));
>> >>  error_tlb:
>> >>  	kfree(pool);
>> >>  error:
>> >> @@ -742,7 +763,9 @@ static void swiotlb_dyn_alloc(struct work_struct *work)
>> >>  	struct io_tlb_pool *pool;
>> >>  
>> >>  	pool = swiotlb_alloc_pool(NULL, IO_TLB_MIN_SLABS, default_nslabs,
>> >> -				  default_nareas, mem->phys_limit, GFP_KERNEL);
>> >> +				  default_nareas, mem->phys_limit,
>> >> +				  mem->unencrypted ? DMA_ATTR_CC_SHARED : 0,
>> >> +				  GFP_KERNEL);
>> >>  	if (!pool) {
>> >>  		pr_warn_ratelimited("Failed to allocate new pool");
>> >>  		return;
>> >> @@ -762,7 +785,7 @@ static void swiotlb_dyn_free(struct rcu_head *rcu)
>> >>  	size_t tlb_size = pool->end - pool->start;
>> >>  
>> >>  	free_pages((unsigned long)pool->slots, get_order(slots_size));
>> >> -	swiotlb_free_tlb(pool->vaddr, tlb_size);
>> >> +	swiotlb_free_tlb(pool->vaddr, tlb_size, pool->unencrypted);
>> >>  	kfree(pool);
>> >>  }
>> >>  
>> >> @@ -1232,6 +1255,7 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
>> >>  	nslabs = nr_slots(alloc_size);
>> >>  	phys_limit = min_not_zero(*dev->dma_mask, dev->bus_dma_limit);
>> >>  	pool = swiotlb_alloc_pool(dev, nslabs, nslabs, 1, phys_limit,
>> >> +				  mem->unencrypted ? DMA_ATTR_CC_SHARED : 0,
>> >>  				  GFP_NOWAIT);
>> >>  	if (!pool)
>> >>  		return -1;
>> >> @@ -1394,6 +1418,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
>> >>  		enum dma_data_direction dir, unsigned long attrs)
>> >>  {
>> >>  	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>> >> +	bool require_decrypted = false;
>> >>  	unsigned int offset;
>> >>  	struct io_tlb_pool *pool;
>> >>  	unsigned int i;
>> >> @@ -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))
>> >> +		require_decrypted = true;
>> >> +
>> >> +	if (require_decrypted != mem->unencrypted)
>> >> +		return (phys_addr_t)DMA_MAPPING_ERROR;
>> >> +
>> >>  	/*
>> >>  	 * The default swiotlb memory pool is allocated with PAGE_SIZE
>> >>  	 * alignment. If a mapping is requested with larger alignment,
>> >> @@ -1608,8 +1643,14 @@ 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);
>> >> +	/*
>> >> +	 * Use the allocated io_tlb_mem encryption type to determine dma addr.
>> >> +	 */
>> >> +	if (dev->dma_io_tlb_mem->unencrypted)
>> >> +		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 +1814,8 @@ static inline void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,
>> >>  
>> >>  #ifdef CONFIG_DMA_RESTRICTED_POOL
>> >>  
>> >> -struct page *swiotlb_alloc(struct device *dev, size_t size)
>> >> +struct page *swiotlb_alloc(struct device *dev, size_t size,
>> >> +		unsigned long attrs)
>> >>  {
>> >>  	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>> >>  	struct io_tlb_pool *pool;
>> >> @@ -1784,6 +1826,9 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
>> >>  	if (!mem)
>> >>  		return NULL;
>> >>  
>> >> +	if (mem->unencrypted != !!(attrs & DMA_ATTR_CC_SHARED))
>> >> +		return NULL;
>> >> +
>> >>  	align = (1 << (get_order(size) + PAGE_SHIFT)) - 1;
>> >>  	index = swiotlb_find_slots(dev, 0, size, align, &pool);
>> >>  	if (index == -1)
>> >> @@ -1853,9 +1898,18 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
>> >>  			kfree(mem);
>> >>  			return -ENOMEM;
>> >>  		}
>> >> +		/*
>> >> +		 * if platform supports memory encryption,
>> >> +		 * restricted mem pool is decrypted by default
>> >> +		 */
>> >> +		if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) {
>> >> +			mem->unencrypted = true;
>> >> +			set_memory_decrypted((unsigned long)phys_to_virt(rmem->base),
>> >> +					     rmem->size >> PAGE_SHIFT);
>> >> +		} else {
>> >> +			mem->unencrypted = false;
>> >> +		}
>> >
>> > This breaks pKVM as it doesn’t set CC_ATTR_MEM_ENCRYPT, so all virtio
>> > traffic now fails.
>> >
>> > Also, by design, some drivers are clueless about bouncing, so
>> > I believe that the pool should have a way to control it’s property
>> > (encrypted or decrypted) and that takes priority over whatever
>> > attributes comes from allocation.
>> > And that brings us to the same point whether it’s better to return
>> > the memory along with it’s state or we pass the requested state.
>> > I think for other cases it’s fine for the device/DMA-API to dictate
>> > the attrs, but not in restricted-dma case, the firmware just knows better.
>> >
>> 
>> Is it that the pKVM guest kernel does not have awareness of
>> encrypted/decrypted DMA allocations? Instead, the firmware attaches
>> hypervisor-shared pages to the device via restricted-dma-pool? The
>> kernel then has swiotlb->for_alloc = true, and hence all DMA allocations
>> go through the restricted-dma-pool?
>
> Yes.
>
>> 
>> Given that pKVM supports pkvm_set_memory_encrypted() and
>> pkvm_set_memory_decrypted(), can we consider adding CC_ATTR_MEM_ENCRYPT
>> support to pKVM? It would also be good to investigate whether we can set
>> force_dma_unencrypted(dev) to true where needed.
>
> I was looking in to that, but it didn't work because
> force_dma_unencrypted() is broken with restricted-dma due to the
> double decryption issue, that's when I sent my first series [1]
>
> May be we should land some basic fixes for that path so we can
> convert pKVM, then we do the full rework.
>
> I will revive my old work and see if I can send a RFC.
>
> [1] https://lore.kernel.org/all/20260305170335.963568-1-smostafa@google.com/
>

With this series, can you check whether the only change needed is
something like the following?

modified   kernel/dma/swiotlb.c
@@ -1905,7 +1905,8 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
 		 * if platform supports memory encryption,
 		 * restricted mem pool is decrypted by default
 		 */
-		if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) {
+		//if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) {
+		if (true) {
 			mem->unencrypted = true;
 			set_memory_decrypted((unsigned long)phys_to_virt(rmem->base),
 					     rmem->size >> PAGE_SHIFT);

>
>> 
>> I agree that this patch, as it stands, can break pKVM because we are now
>> missing the set_memory_decrypted() call required for pKVM to work.
>> 
>> We now mark the swiotlb io_tlb_mem as unencrypted/encrypted in the guest
>> using struct io_tlb_mem->unencrypted. I am not clear what we can use for
>> pKVM to conditionalize this so that it works for both protected and
>> unprotected guests.
>
> There is no problem with non-protected guests as they don't use memory
> encryption, my initial thought was that th encrpyted/decrypted is
> per-pool property which is decided by FW (device-tree).
>

What I meant was that we need a generic way to identify a pKVM guest, so
that we can use it in the conditional above.

-aneesh