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Re: [Xen-devel] DMA trouble with current xen-sparse Keir Fraser Sat Oct 29 02:00:47 2005


On 28 Oct 2005, at 20:21, Stephen C. Tweedie wrote:

The trouble is that this is a 1G box, so its memory is not large enough
to automatically enable the swiotlb.  (arch/xen/i386/kernel/swiotlb.c
enables swiotlb automatically for dom0 only if there's at least 2G of
memory.)  And the first time we get a pci_dma_single() request for a
dom0-contiguous region which crosses a page boundary, we hit the BUG_ON
at arch/xen/i386/kernel/pci_dma.c:270 due to dma_map_single() checking:

                IOMMU_BUG_ON(range_straddles_page_boundary(ptr, size));

And this happens *instantly* on any loaded tcp connection on my e1000
NIC.  All I need to do to kill the box is to ssh in and type "find\n".
Instant dom0 death after the ssh client receives about a dozen lines of
output.  The stack trace is appended below.

Is the network interface set up to use jumbo frames? Otherwise I wouldn't expect alloc_skb() to allocate a data area that straddles a page boundary, since the allocation will come from one of the sub-page-sized power-of-two kmem caches.

If the problem is jumbo frames, we might need to add a hook to alloc_skb(). Using swiotlb will suck hugely.

 -- Keir


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