linux-kernel - [PATCH v4 4/5] swiotlb: Fix alignment checks when both allocation and DMA masks are present

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Message-Id: <20240221113504.7161-5-will@kernel.org>
Date: Wed, 21 Feb 2024 11:35:03 +0000
From: Will Deacon <will@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: kernel-team@...roid.com,
	Will Deacon <will@...nel.org>,
	iommu@...ts.linux.dev,
	Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Dexuan Cui <decui@...rosoft.com>,
	Nicolin Chen <nicolinc@...dia.com>,
	Michael Kelley <mhklinux@...look.com>
Subject: [PATCH v4 4/5] swiotlb: Fix alignment checks when both allocation and DMA masks are present

Nicolin reports that swiotlb buffer allocations fail for an NVME device
behind an IOMMU using 64KiB pages. This is because we end up with a
minimum allocation alignment of 64KiB (for the IOMMU to map the buffer
safely) but a minimum DMA alignment mask corresponding to a 4KiB NVME
page (i.e. preserving the 4KiB page offset from the original allocation).
If the original address is not 4KiB-aligned, the allocation will fail
because swiotlb_search_pool_area() erroneously compares these unmasked
bits with the 64KiB-aligned candidate allocation.

Tweak swiotlb_search_pool_area() so that the DMA alignment mask is
reduced based on the required alignment of the allocation.

Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
Reported-by: Nicolin Chen <nicolinc@...dia.com>
Link: https://lore.kernel.org/r/cover.1707851466.git.nicolinc@nvidia.com
Signed-off-by: Will Deacon <will@...nel.org>
---
 kernel/dma/swiotlb.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index c20324fba814..c381a7ed718f 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -981,8 +981,7 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
 	dma_addr_t tbl_dma_addr =
 		phys_to_dma_unencrypted(dev, pool->start) & boundary_mask;
 	unsigned long max_slots = get_max_slots(boundary_mask);
-	unsigned int iotlb_align_mask =
-		dma_get_min_align_mask(dev) & ~(IO_TLB_SIZE - 1);
+	unsigned int iotlb_align_mask = dma_get_min_align_mask(dev);
 	unsigned int nslots = nr_slots(alloc_size), stride;
 	unsigned int offset = swiotlb_align_offset(dev, orig_addr);
 	unsigned int index, slots_checked, count = 0, i;
@@ -993,6 +992,14 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
 	BUG_ON(!nslots);
 	BUG_ON(area_index >= pool->nareas);
 
+	/*
+	 * Ensure that the allocation is at least slot-aligned and update
+	 * 'iotlb_align_mask' to ignore bits that will be preserved when
+	 * offsetting into the allocation.
+	 */
+	alloc_align_mask |= (IO_TLB_SIZE - 1);
+	iotlb_align_mask &= ~alloc_align_mask;
+
 	/*
 	 * For mappings with an alignment requirement don't bother looping to
 	 * unaligned slots once we found an aligned one.
-- 
2.44.0.rc0.258.g7320e95886-goog