linux-kernel - [PATCH v2 1/2] xen/swiotlb: relax alignment requirements

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Message-ID: <20250213085538.17060-2-jgross@suse.com>
Date: Thu, 13 Feb 2025 09:55:37 +0100
From: Juergen Gross <jgross@...e.com>
To: linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev
Cc: Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	xen-devel@...ts.xenproject.org,
	Jan Vejvalka <jan.vejvalka@...otol.cuni.cz>
Subject: [PATCH v2 1/2] xen/swiotlb: relax alignment requirements

When mapping a buffer for DMA via .map_page or .map_sg DMA operations,
there is no need to check the machine frames to be aligned according
to the mapped areas size. All what is needed in these cases is that the
buffer is contiguous at machine level.

So carve out the alignment check from range_straddles_page_boundary()
and move it to a helper called by xen_swiotlb_alloc_coherent() and
xen_swiotlb_free_coherent() directly.

Fixes: 9f40ec84a797 ("xen/swiotlb: add alignment check for dma buffers")
Reported-by: Jan Vejvalka <jan.vejvalka@...otol.cuni.cz>
Tested-by: Jan Vejvalka <jan.vejvalka@...otol.cuni.cz>
Signed-off-by: Juergen Gross <jgross@...e.com>
Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>
---
 drivers/xen/swiotlb-xen.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index a337edcf8faf..26c62e0d34e9 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -74,19 +74,21 @@ static inline phys_addr_t xen_dma_to_phys(struct device *dev,
 	return xen_bus_to_phys(dev, dma_to_phys(dev, dma_addr));
 }
 
+static inline bool range_requires_alignment(phys_addr_t p, size_t size)
+{
+	phys_addr_t algn = 1ULL << (get_order(size) + PAGE_SHIFT);
+	phys_addr_t bus_addr = pfn_to_bfn(XEN_PFN_DOWN(p)) << XEN_PAGE_SHIFT;
+
+	return IS_ALIGNED(p, algn) && !IS_ALIGNED(bus_addr, algn);
+}
+
 static inline int range_straddles_page_boundary(phys_addr_t p, size_t size)
 {
 	unsigned long next_bfn, xen_pfn = XEN_PFN_DOWN(p);
 	unsigned int i, nr_pages = XEN_PFN_UP(xen_offset_in_page(p) + size);
-	phys_addr_t algn = 1ULL << (get_order(size) + PAGE_SHIFT);
 
 	next_bfn = pfn_to_bfn(xen_pfn);
 
-	/* If buffer is physically aligned, ensure DMA alignment. */
-	if (IS_ALIGNED(p, algn) &&
-	    !IS_ALIGNED((phys_addr_t)next_bfn << XEN_PAGE_SHIFT, algn))
-		return 1;
-
 	for (i = 1; i < nr_pages; i++)
 		if (pfn_to_bfn(++xen_pfn) != ++next_bfn)
 			return 1;
@@ -156,7 +158,8 @@ xen_swiotlb_alloc_coherent(struct device *dev, size_t size,
 
 	*dma_handle = xen_phys_to_dma(dev, phys);
 	if (*dma_handle + size - 1 > dma_mask ||
-	    range_straddles_page_boundary(phys, size)) {
+	    range_straddles_page_boundary(phys, size) ||
+	    range_requires_alignment(phys, size)) {
 		if (xen_create_contiguous_region(phys, order, fls64(dma_mask),
 				dma_handle) != 0)
 			goto out_free_pages;
@@ -182,7 +185,8 @@ xen_swiotlb_free_coherent(struct device *dev, size_t size, void *vaddr,
 	size = ALIGN(size, XEN_PAGE_SIZE);
 
 	if (WARN_ON_ONCE(dma_handle + size - 1 > dev->coherent_dma_mask) ||
-	    WARN_ON_ONCE(range_straddles_page_boundary(phys, size)))
+	    WARN_ON_ONCE(range_straddles_page_boundary(phys, size) ||
+			 range_requires_alignment(phys, size)))
 	    	return;
 
 	if (TestClearPageXenRemapped(virt_to_page(vaddr)))
-- 
2.43.0