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

[Jürgen Groß]

On 16.09.24 08:59, Jan Beulich wrote:

On 16.09.2024 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);

+       if (IS_ALIGNED(p, algn) &&
+           !IS_ALIGNED(next_bfn << XEN_PAGE_SHIFT, algn))

next_bfn = pfn_to_bfn(xen_pfn); + /* If buffer is physically aligned, ensure DMA alignment. */

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).

Oh, yes - of course. A (hypothetical?) strict no-overflow checking
mode of the kernel may take issue though, so maybe better to right-
shift "algn"?

No, this wouldn't work in case algn < XEN_PAGE_SIZE.


Juergen

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