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

[Matt Wilson <msw@xxxxxxxxx>]

On Wed, Mar 26, 2014 at 11:13:49AM +0000, Ian Campbell wrote:
> On Wed, 2014-03-26 at 12:48 +0200, Matt Wilson wrote:
> > On Wed, Mar 26, 2014 at 10:44:18AM +0000, David Vrabel wrote:
> > > On 26/03/14 10:17, Matt Wilson wrote:
> > > > 
> > > > Konrad, what's the possibility of fixing this in Linux Xen PV setup
> > > > code? I think it'd be a matter batching up pages and doing larger
> > > > order allocations in linux/arch/x86/xen/setup.c:xen_do_chunk(),
> > > > falling back to smaller pages if allocations fail due to
> > > > fragmentation, etc.
> > > 
> > > We plan to fix problems caused by non-machine-contiguous memory by
> > > setting up the IOMMU to have 1:1 bus to pseudo-physical mappings.  This
> > > would avoid using the swiotlb always[1], regardless of the machine
> > > layout of dom0 or the driver domain.
> > > 
> > > I think I would prefer this approach rather than making xen/setup.c even
> > > more horribly complicated.
> > 
> > I imagine that some users will not want to run dom0 under an IOMMU.
> 
> Then they have chosen that (for whatever reason) over performance,
> right?

IOMMU is not free, so I imagine that some users would actually be
choosing to avoid it specifically for performance reasons.

--msw

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