Re: [PATCH v3 4/4] dma: swiotlb: Remove SWIOTLB_ANY

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Inverting the default SWIOTLB allocation behavior from `memblock_alloc_low()` to `memblock_alloc()` breaks DMA for legacy 32-bit devices on x86 systems with `iommu=off` when memory encryption or `swiotlb=force` is used.
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commit 35955ff88ad01e61df3c901065f4e509abaab7d0
Author: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>

dma: swiotlb: Remove SWIOTLB_ANY

SWIOTLB_ANY permits the swiotlb pool to be allocated outside the
architecture's low address limit. This is already implied when
SWIOTLB_INIT_ADDRESSING_LIMIT is not set, making SWIOTLB_ANY redundant.

Use SWIOTLB_INIT_ADDRESSING_LIMIT as the sole indication that the
default pool and any dynamically allocated pools must reside below
ARCH_LOW_ADDRESS_LIMIT. Otherwise, allow the pools to use any directly
mapped memory.

> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c4611b2c45403..2715833a21f8a 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -448,15 +448,10 @@ 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);

[Severity: High]
Does inverting the default SWIOTLB allocation behavior from memblock_alloc_low()
to memblock_alloc() break DMA for legacy 32-bit devices on x86 systems?

If a system boots with iommu=off and swiotlb=force (or memory encryption),
pci_swiotlb_detect() leaves x86_swiotlb_flags as 0 because no_iommu is true:

arch/x86/kernel/pci-dma.c:pci_swiotlb_detect() {
	/* don't initialize swiotlb if iommu=off (no_iommu=1) */
	if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN) {
		x86_swiotlb_enable = true;
		x86_swiotlb_flags |= SWIOTLB_INIT_ADDRESSING_LIMIT;
	}
    ...
}

Because SWIOTLB_INIT_ADDRESSING_LIMIT is not set, this new logic uses
memblock_alloc() instead of the previous memblock_alloc_low(), placing the
bounce buffer pool anywhere, including above 4GB.

Legacy 32-bit devices will then fail DMA mappings because they receive bounce
buffers allocated above 4GB, failing dma_capable() checks and causing
swiotlb_map() to return DMA_MAPPING_ERROR.

[ ... ]

> @@ -598,10 +593,10 @@ 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

[Severity: High]
Similarly, does this incorrectly record an unrestricted phys_limit for the pool
when SWIOTLB_INIT_ADDRESSING_LIMIT is missing under the same iommu=off
conditions?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260904101810.1193505-1-aneesh.kumar@kernel.org?part=4