[PATCH v5 6/6] dma: swiotlb: Remove SWIOTLB_ANY
Robin Murphy
robin.murphy at arm.com
Wed Sep 23 06:12:21 PDT 2026
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 at arm.com>
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar at 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 */
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