linux-kernel - [PATCH 3/3] swiotlb: ensure a segment doesn't cross the area boundary

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Message-Id: <20220715104535.1053907-4-chao.gao@intel.com>
Date:   Fri, 15 Jul 2022 18:45:35 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Cc:     Chao Gao <chao.gao@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>
Subject: [PATCH 3/3] swiotlb: ensure a segment doesn't cross the area boundary

Free slots tracking assumes that slots in a segment can be allocated to
fulfill a request. This implies that slots in a segment should belong to
the same area. Although the possibility of a violation is low, it is better
to explicitly enforce segments won't span multiple areas by adjusting the
number of slabs when configuring areas.

Signed-off-by: Chao Gao <chao.gao@...el.com>
---
 kernel/dma/swiotlb.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 9ab87d6d47bc..70fd73fc357a 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -91,12 +91,21 @@ struct io_tlb_area {
 /*
  * Round up number of slabs to the next power of 2. The last area is going
  * be smaller than the rest if default_nslabs is not power of two.
+ * The number of slot in an area should be a multiple of IO_TLB_SEGSIZE,
+ * otherwise a segment may span two or more areas. It conflicts with free
+ * contiguous slots tracking: free slots are treated contiguous no matter
+ * whether they cross an area boundary.
  *
  * Return true if default_nslabs is rounded up.
  */
 static bool round_up_default_nslabs(void)
 {
-	if (!default_nareas || is_power_of_2(default_nslabs))
+	if (!default_nareas)
+		return false;
+
+	if (default_nslabs < IO_TLB_SEGSIZE * default_nareas)
+		default_nslabs = IO_TLB_SEGSIZE * default_nareas;
+	else if (is_power_of_2(default_nslabs))
 		return false;
 
 	default_nslabs = roundup_pow_of_two(default_nslabs);
-- 
2.25.1