Re: [Xen-devel] [PATCH 03 of 38] swiotlb: allow architectures tooverride swiotlb pool allocation

[Chris Lalancette <clalance@xxxxxxxxxx>]

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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