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


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"

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?


Julien Grall

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