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

[David Vrabel <david.vrabel@citrix.com>]

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...

I'm not ruling out any Linux-side memory setup changes.  I just don't
think they're a complete solution.

>> Malcolm has a working prototype of this already.
> 
> Is it ready for a RFC posting?

Not sure. Malcolm is current OoO.

David