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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 26, 2014 at 08:59:04AM -0700, Matthew Rushton wrote:
> 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.

Large size like larger than 512kB? That would also cause problems
on baremetal then when swiotlb is activated I believe.

> >
> >--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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