Re: [Xen-devel] [RFC] xen/arm: Handling cache maintenance instructions by set/way

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

On 12/10/2017 03:22 PM, Tim Deegan wrote:
> At 14:38 +0000 on 08 Dec (1512743913), Julien Grall wrote:
> > On 08/12/17 08:03, Tim Deegan wrote:
> > > +1 for avoiding the full majesty of PoD if you don't need it.
> > >
> > > It should be possible to do something like the misconfigured-entry bit
> > > trick by _allocating_ the memory up-front and building the p2m entries
> > > but only making them usable by the {IO}MMUs on first access.  That
> > > would make these early p2m walks shorter (because they can skip whole
> > > subtrees that aren't marked present yet) without making major changes
> > > to domain build or introducing run-time failures.
> > I am not aware of any way on Arm to misconfigure an entry. We do have
> > valid and access bits, although they will affect the IOMMU as well. So
> > it will not be possible to get page-table sharing with this "feature"
> > enabled.
> How unfortunate.  How does KVM's demand-population scheme handle the IOMMU?
From what I have heard, when using IOMMU all the memory is pinned. They 
also don't share page-tables.
But I am not a KVM expert, maybe Andre/Marc can confirm here?

Cheers,

--
Julien Grall

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