Re: [Xen-devel] [RFC PATCH] page_alloc: use first half of higher order chunks when halving

[Matthew Rushton <mvrushton@xxxxxxxxx>]

On 03/26/14 08:15, Matt Wilson wrote:
> On Wed, Mar 26, 2014 at 11:08:01AM -0400, Konrad Rzeszutek Wilk wrote:
> > Could you elaborate a bit more on the use-case please?
> > My understanding is that most drivers use a scatter gather list - in which
> > case it does not matter if the underlaying MFNs in the PFNs spare are
> > not contingous.
> >
> > But I presume the issue you are hitting is with drivers doing dma_map_page
> > and the page is not 4KB but rather large (compound page). Is that the
> > problem you have observed?
> Drivers are using very large size arguments to dma_alloc_coherent()
> for things like RX and TX descriptor rings.
>
> --msw
It's the dma streaming api I've noticed the problem with, so 
dma_map_single(). Applicable swiotlb code would be 
xen_swiotlb_map_page() and range_straddles_page_boundary(). So yes for 
larger buffers it can cause bouncing.
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