Re: [PATCH v5 6/6] dma: swiotlb: Remove SWIOTLB_ANY

[Robin Murphy <robin.murphy@arm.com>]

On 21/09/2026 7:36 am, Aneesh Kumar K.V (Arm) wrote:
> swiotlb pools are currently allocated below the architecture's low
> address limit by default. SWIOTLB_ANY overrides this and permits
> allocation anywhere in directly mapped memory.
> 
> SWIOTLB_INIT_ADDRESSING_LIMIT now explicitly identifies callers which
> need a pool that is reachable by devices with limited DMA addressing.
> Use it as the placement constraint and remove SWIOTLB_ANY.
> 
> This changes the default behavior for callers which do not set either
> flag. Previously, omitting SWIOTLB_ANY placed the default pool below the
> architecture's low address limit and also constrained dynamically
> allocated pools to the same range. After this change, omitting
> SWIOTLB_INIT_ADDRESSING_LIMIT permits both static and dynamic pools to
> use any directly mapped memory.

While it's rather dependent on some of the other aspects wanting a bit 
more work still, this part I do entirely agree with :)

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>   Documentation/core-api/swiotlb.rst |  8 ++++----
>   arch/powerpc/mm/mem.c              |  6 ++++--
>   arch/riscv/mm/init.c               | 16 ----------------
>   arch/s390/mm/init.c                |  2 +-
>   arch/x86/kernel/pci-dma.c          |  2 +-
>   include/linux/swiotlb.h            |  7 +++----
>   kernel/dma/swiotlb.c               | 17 ++++++-----------
>   7 files changed, 19 insertions(+), 39 deletions(-)
> 
> diff --git a/Documentation/core-api/swiotlb.rst b/Documentation/core-api/swiotlb.rst
> index 71b4e4c27eb5..06d9f88fdf7b 100644
> --- a/Documentation/core-api/swiotlb.rst
> +++ b/Documentation/core-api/swiotlb.rst
> @@ -148,10 +148,10 @@ may also be adjusted
>   due to other conditions, such as running in a CoCo VM, as described above. If
>   CONFIG_SWIOTLB_DYNAMIC is enabled, additional pools may be allocated later in
>   the life of the system. Each pool must be a contiguous range of physical
> -memory. The default pool is allocated below the 4 GiB physical address line so
> -it works for devices that can only address 32-bits of physical memory (unless
> -architecture-specific code provides the SWIOTLB_ANY flag). In a CoCo VM, the
> -pool memory must be decrypted before swiotlb is used.
> +memory. The default pool is allocated below the architecture's low address
> +limit when it is needed for devices with limited DMA addressing. Otherwise,
> +it may be allocated anywhere in directly mapped memory. In a CoCo VM, the pool
> +memory must be decrypted before swiotlb is used.
>   
>   Each pool is divided into "slots" of size IO_TLB_SIZE, which is 2 KiB with
>   current definitions. IO_TLB_SEGSIZE contiguous slots (128 slots) constitute
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 006ed41097e9..1bdea6c03848 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -295,9 +295,11 @@ void __init arch_mm_preinit(void)
>   		/*
>   		 * Since the guest memory is inaccessible to the host,
>   		 * devices always need to use the SWIOTLB buffer for DMA
> -		 * even if dma_capable() says otherwise.
> +		 * even if dma_capable() says otherwise. The hypervisor has
> +		 * no addressing limitation, so the buffer may be allocated
> +		 * anywhere.
>   		 */
> -		ppc_swiotlb_flags |= SWIOTLB_ANY;
> +		ppc_swiotlb_flags &= ~SWIOTLB_INIT_ADDRESSING_LIMIT;
>   	}
>   
>   	/*
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 96856114b387..734712230f35 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -165,27 +165,11 @@ static void print_vm_layout(void) { }
>   
>   void __init arch_mm_preinit(void)
>   {
> -	bool swiotlb = max_pfn > PFN_DOWN(dma32_phys_limit) &&
> -		       memblock_start_of_DRAM() < dma32_phys_limit;
>   	unsigned int swiotlb_flags = SWIOTLB_VERBOSE;
>   #ifdef CONFIG_FLATMEM
>   	BUG_ON(!mem_map);
>   #endif /* CONFIG_FLATMEM */
>   
> -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb &&
> -	    dma_cache_alignment != 1) {
> -		/*
> -		 * No 32-bit DMA bouncing needed (either all DRAM is within
> -		 * the 32-bit limit, or it all starts above it), but
> -		 * kmalloc() buffers whose sizes are not cache-line-aligned
> -		 * still require bouncing for non-coherent DMA.  Use
> -		 * SWIOTLB_ANY so that the buffer can be allocated from high
> -		 * memory when DRAM starts above dma32_phys_limit.  Allocate
> -		 * ~1 MB per 1 GB of RAM.
> -		 */
> -		swiotlb_flags |= SWIOTLB_ANY;
> -	}
> -
>   	if ((max_pfn > PFN_DOWN(dma32_phys_limit)) &&
>   	     (memblock_start_of_DRAM() < dma32_phys_limit))
>   		swiotlb_flags |= SWIOTLB_INIT_ADDRESSING_LIMIT;
> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> index 78f507e24ceb..ce10292447f1 100644
> --- a/arch/s390/mm/init.c
> +++ b/arch/s390/mm/init.c
> @@ -166,7 +166,7 @@ static void __init pv_init(void)
>   	virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc);
>   
>   	/* make sure bounce buffers are shared */
> -	swiotlb_init(SWIOTLB_VERBOSE | SWIOTLB_ANY);
> +	swiotlb_init(SWIOTLB_VERBOSE);
>   	swiotlb_update_mem_attributes();
>   }
>   
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index ce257b20ea9c..74e92b457758 100644
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -90,7 +90,7 @@ static void __init pci_xen_swiotlb_init(void)
>   	/* Xen can use a SWIOTLB pool anywhere in directly mapped memory. */
>   	x86_swiotlb_flags &= ~(SWIOTLB_INIT_ADDRESSING_LIMIT |
>   			       SWIOTLB_INIT_DEFAULT_OFF);
> -	x86_swiotlb_flags |= SWIOTLB_INIT_REMAP | SWIOTLB_ANY;
> +	x86_swiotlb_flags |= SWIOTLB_INIT_REMAP;
>   	swiotlb_init_remap(x86_swiotlb_flags, xen_swiotlb_fixup);
>   	dma_ops = &xen_swiotlb_dma_ops;
>   	if (IS_ENABLED(CONFIG_PCI))
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 466c33795db0..503e1446e0d8 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -15,13 +15,12 @@ struct page;
>   struct scatterlist;
>   
>   #define SWIOTLB_VERBOSE	(1 << 0) /* verbose initialization */
> -#define SWIOTLB_ANY	(1 << 1) /* allow any memory for the buffer */
>   /* Initialize a default-sized pool for devices with limited DMA addressing. */
> -#define SWIOTLB_INIT_ADDRESSING_LIMIT	(1 << 2)
> +#define SWIOTLB_INIT_ADDRESSING_LIMIT	(1 << 1)
>   /* Initialize a default-sized pool that requires architecture remapping. */
> -#define SWIOTLB_INIT_REMAP		(1 << 3)
> +#define SWIOTLB_INIT_REMAP		(1 << 2)
>   /* Do not initialize a pool unless SWIOTLB is explicitly required. */
> -#define SWIOTLB_INIT_DEFAULT_OFF		(1 << 4)
> +#define SWIOTLB_INIT_DEFAULT_OFF	(1 << 3)
>   
>   /*
>    * Maximum allowable number of contiguous slabs to map,
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 46b789eb0806..77d7c320de28 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -450,15 +450,10 @@ static void __init *swiotlb_memblock_alloc(unsigned long nslabs,
>   	size_t bytes = PAGE_ALIGN(nslabs << IO_TLB_SHIFT);
>   	void *tlb;
>   
> -	/*
> -	 * By default allocate the bounce buffer memory from low memory, but
> -	 * allow to pick a location everywhere for hypervisors with guest
> -	 * memory encryption.
> -	 */
> -	if (flags & SWIOTLB_ANY)
> -		tlb = memblock_alloc(bytes, PAGE_SIZE);
> -	else
> +	if (flags & SWIOTLB_INIT_ADDRESSING_LIMIT)
>   		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
> +	else
> +		tlb = memblock_alloc(bytes, PAGE_SIZE);
>   
>   	if (!tlb) {
>   		pr_warn("%s: Failed to allocate %zu bytes tlb structure\n",
> @@ -604,10 +599,10 @@ void __init swiotlb_init_remap(unsigned int flags,
>   #ifdef CONFIG_SWIOTLB_DYNAMIC
>   	if (!remap)
>   		io_tlb_default_mem.can_grow = true;
> -	if (flags & SWIOTLB_ANY)
> -		io_tlb_default_mem.phys_limit = virt_to_phys(high_memory - 1);
> -	else
> +	if (flags & SWIOTLB_INIT_ADDRESSING_LIMIT)
>   		io_tlb_default_mem.phys_limit = ARCH_LOW_ADDRESS_LIMIT;
> +	else
> +		io_tlb_default_mem.phys_limit = virt_to_phys(high_memory - 1);
>   #endif
>   
>   	/* if we have host or guest memory encryption */