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Re: [PATCH v2 1/2] xen/swiotlb: add alignment check for dma buffers


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Mon, 16 Sep 2024 08:59:45 +0200
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, iommu@xxxxxxxxxxxxxxx
  • Delivery-date: Mon, 16 Sep 2024 06:59:50 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 16.09.24 08:56, Juergen Gross wrote:
On 16.09.24 08:50, Jan Beulich wrote:
On 16.09.2024 08:47, Juergen Gross wrote:
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -78,9 +78,15 @@ 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(next_bfn << XEN_PAGE_SHIFT, algn))

And this shift is not at risk of losing bits on Arm LPAE?

For alignment check this just doesn't matter (assuming XEN_PAGE_SIZE is
smaller than 4G).

Wait, that was nonsense.

I'll change the check to:

        !IS_ALIGNED((phys_addr_t)next_bfn << XEN_PAGE_SHIFT, algn)


Juergen

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