Messages in this thread | | | Date | Mon, 17 Nov 2008 09:04:23 +0100 | From | Chris Lalancette <> | Subject | Re: [Xen-devel] [PATCH 03 of 38] swiotlb: allow architectures tooverride swiotlb pool allocation |
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Jeremy Fitzhardinge wrote: >> While perhaps less intrusive to take care of, I also didn't see an equivalent >> of the range_straddles_page_boundary() logic, without which I can't see >> how this would work in the common case. >> > Could you be more specific? The swiotlb allocation should be machine > contiguous and so there's no stradding required, but I think I'm missing > your point.
In general, I think you are right; swiotlb should be machine contiguous, so it works in the normal case. The range_straddles_page_boundary function takes care of a corner case, where you can run into swiotlb exhaustion when you really shouldn't. As I understand it, it comes about because it is possible to get a swiotlb request with two pages that just happen to be machine contiguous, but were *not* allocated through xen_create_contiguous_region (and hence weren't marked in the contiguous_bitmap as such). In this case, you split the request into two separate requests, and this can more easily lead to exhaustion. range_straddles_page_boundary works around this by checking whether any two pages coming through the swiotlb layer are machine contiguous, and if they are, not splitting the request.
-- Chris Lalancette
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