Re: [Xen-devel] SWIOMMU redundant copies?

["Stephen C. Tweedie" <sct@xxxxxxxxxx>]

Hi,

On Fri, 2008-03-14 at 16:26 +0000, Keir Fraser wrote:

> Yes, the swiotlb check could be relaxed. Do we really do many mergeable
> page-straddling I/Os in practice though? I would expect most block I/O to be
> scattered by the page cache.

I would, too --- but it appears to be a genuine issue at boot time at
least, when the page cache has not yet become too fragmented.

I'm still trying to understand the corner cases involved.  Some
hardware seems to have more trouble than others --- it could well be
special-case sg segments such as DMA drain buffers which are causing the
trouble for those, in which case doing the copy for every map_sg() will
potentially be a significant problem.  (MarkMC hit that same problem
bringing the dom0 patches onto 2.6.25 which has reworked drain buffer
code.)

--Stephen



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