Attachment 299461 Details for Bug 433554 – Detect physically-contiguous pages when determining if memory spans a page boundary

[patch] Detect physically-contiguous pages when determining if memory spans a page boundary

linux-2.6-xen-fix-swiommu-page-spans.patch (text/plain), 3.61 KB, created by Stephen Tweedie on 2008-03-28 13:10:55 UTC

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Creator: Stephen Tweedie

Created: 2008-03-28 13:10:55 UTC

Size: 3.61 KB

patch

obsolete

>commit 952c63d05820a0ac730a8e0a6d902df5cafd6aff
>Author: Stephen Tweedie <sct@redhat.com>
>Date:   Thu Mar 13 17:49:28 2008 +0000
>
>    xen dma: avoid unnecessarily SWIOTLB bounce buffering.
>    
>    On Xen kernels, BIOVEC_PHYS_MERGEABLE permits merging of disk IOs that
>    span multiple pages, provided that the pages are both pseudophysically-
>    AND machine-contiguous ---
>    
>    	(((bvec_to_phys((vec1)) + (vec1)->bv_len) == bvec_to_phys((vec2))) && \
>    	 ((bvec_to_pseudophys((vec1)) + (vec1)->bv_len) == \
>    	  bvec_to_pseudophys((vec2))))
>    
>    However, this best-effort merging of adjacent pages can occur in
>    regions of dom0 memory which just happen, by virtue of having been
>    initially set up that way, to be machine-contiguous.  Such pages
>    which occur outside of a range created by xen_create_contiguous_
>    region won't be seen as contiguous by range_straddles_page_boundary(),
>    so the pci-dma-xen.c code for dma_map_sg() will send these regions
>    to the swiotlb for bounce buffering.
>    
>    In RHEL-5.1 this did not happen, because we did not have the check
>    for range_straddles_page_boundary() in that code.  Now that that check
>    has been added, these SG ranges --- which ARE machine contiguous and
>    which can perfectly well be sent to a dma engine --- are being bounce-
>    buffered in the swiotlb instead, causing a performance overhead and
>    potentially leading to early swiotlb exhaustion.
>    
>    This patch adds a new check, check_pages_physically_contiguous(),
>    to the test for pages stradding page boundaries both in
>    swiotlb_map_sg() and dma_map_sg(), to capture these ranges and map
>    them directly via virt_to_bus() mapping rather than through the
>    swiotlb.
>    
>    Signed-off-by: Stephen Tweedie <sct@redhat.com>
>
>diff --git a/arch/i386/kernel/pci-dma-xen.c b/arch/i386/kernel/pci-dma-xen.c
>index cdeda5a..14f3539 100644
>--- a/arch/i386/kernel/pci-dma-xen.c
>+++ b/arch/i386/kernel/pci-dma-xen.c
>@@ -110,6 +110,39 @@ do {							\
> 	}						\
> } while (0)
> 
>+static int check_pages_physically_contiguous(unsigned long pfn, 
>+					     unsigned int offset,
>+					     size_t length)
>+{
>+	unsigned long next_mfn;
>+	int i;
>+	int nr_pages;
>+	
>+	next_mfn = pfn_to_mfn(pfn);
>+	nr_pages = (offset + length + PAGE_SIZE-1) >> PAGE_SHIFT;
>+	
>+	for (i = 1; i < nr_pages; i++) {
>+		if (pfn_to_mfn(++pfn) != ++next_mfn) 
>+			return 0;
>+	}
>+	return 1;
>+}
>+
>+int range_straddles_page_boundary(paddr_t p, size_t size)
>+{
>+	extern unsigned long *contiguous_bitmap;
>+	unsigned long pfn = p >> PAGE_SHIFT;
>+	unsigned int offset = p & ~PAGE_MASK;
>+
>+	if (offset + size <= PAGE_SIZE)
>+		return 0;
>+	if (test_bit(pfn, contiguous_bitmap))
>+		return 0;
>+	if (check_pages_physically_contiguous(pfn, offset, size))
>+		return 0;
>+	return 1;
>+}
>+
> int
> dma_map_sg(struct device *hwdev, struct scatterlist *sg, int nents,
> 	   enum dma_data_direction direction)
>diff --git a/include/asm-i386/mach-xen/asm/dma-mapping.h b/include/asm-i386/mach-xen/asm/dma-mapping.h
>index 18b1a0d..fc917f2 100644
>--- a/include/asm-i386/mach-xen/asm/dma-mapping.h
>+++ b/include/asm-i386/mach-xen/asm/dma-mapping.h
>@@ -22,13 +22,7 @@ address_needs_mapping(struct device *hwdev, dma_addr_t addr)
> 	return (addr & ~mask) != 0;
> }
> 
>-static inline int
>-range_straddles_page_boundary(paddr_t p, size_t size)
>-{
>-	extern unsigned long *contiguous_bitmap;
>-	return ((((p & ~PAGE_MASK) + size) > PAGE_SIZE) &&
>-		!test_bit(p >> PAGE_SHIFT, contiguous_bitmap));
>-}
>+extern int range_straddles_page_boundary(paddr_t p, size_t size);
> 
> #define dma_alloc_noncoherent(d, s, h, f) dma_alloc_coherent(d, s, h, f)
> #define dma_free_noncoherent(d, s, v, h) dma_free_coherent(d, s, v, h)

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Attachments on bug 433554: 295354 | 295355 | 295526 | 298053 | 299461