Re: [Xen-devel] [PATCH v6 13/14] x86: add iommu_ops to modify and flush IOMMU mappings

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Paul Durrant
> Sent: Wednesday, September 12, 2018 4:02 PM
> 
> 
> > In order to avoid shooting down all pre-existing RAM mappings - is
> > there no way the page table entries could be marked to identify
> > their origin?
> >
> 
> I don't know whether that is possible; I'll have to find specs for Intel and
> AMD IOMMUs and see if they have PTE bits available for such a use.

there are ignored bits

> 
> > I also have another more general concern: Allowing the guest to
> > manipulate its IOMMU page tables means that it can deliberately
> > shatter large pages, growing the overall memory footprint of the
> > domain. I'm hesitant to say this, but I'm afraid that resource
> > tracking of such "behind the scenes" allocations might be a
> > necessary prereq for the PV IOMMU work.
> >
> 
> Remember that PV-IOMMU is only available for dom0 as it stands (and that
> is the only use-case that XenServer currently has) so I think that, whilst the
> concern is valid, there is no need danger in putting the code without such
> tracking. Such work can be deferred to making PV-IOMMU for de-privileged
> guests... if that facility is needed.
> 

I didn't get why this is PV-IOMMU specific. Guest can always manipulate
guest CPU page table to shatter large pages too...

Thanks
Kevin
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