Re: [Xen-devel] Load increase after memory upgrade (part2)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2011-11-28 at 15:28 +0000, Konrad Rzeszutek Wilk wrote:
> On Fri, Nov 25, 2011 at 11:11:55PM +0100, Carsten Schiers wrote:

> > I looked through my old mails from you and you explained already the 
> > necessity of double
> > bounce buffering (PCI->below 4GB->above 4GB). What I don't understand is: 
> > why does the
> > Xenified kernel not have this kind of issue?
> 
> That is a puzzle. It should not. The code is very much the same - both
> use the generic SWIOTLB which has not changed for years.

The swiotlb-xen used by classic-xen kernels (which I assume is what
Carsten means by "Xenified") isn't exactly the same as the stuff in
mainline Linux, it's been heavily refactored for one thing. It's not
impossible that mainline is bouncing something it doesn't really need
to.

It's also possible that the dma mask of the device is different/wrong in
mainline leading to such additional bouncing.

I guess it's also possible that the classic-Xen kernels are playing fast
and loose by not bouncing something they should (although if so they
appear to be getting away with it...) or that there is some difference
which really means mainline needs to bounce while classic-Xen doesn't.

Ian.



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