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 12:43:25PM +0000, David Vrabel wrote:
> On 26/03/14 10:48, 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. If
> > changing Linux Xen PV setup is (rightly) objectionable due to
> > complexity, perhaps this small change to the hypervisor is a better
> > short-term fix.
> 
> Users who are not using the IOMMU for performance reasons but are
> complaining about swiotlb costs?  I'm not sure that's an interesting set
> of users...

You have to admit that a configuration that avoids both IOMMU and
bouncing in swiotlb will be the best performance in many scenarios,
don't you think?

--msw

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