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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Tian, Kevin [mailto:kevin.tian@xxxxxxxxx]
> Sent: 13 September 2018 07:41
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Jan Beulich'
> <JBeulich@xxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Tim (Xen.org)
> <tim@xxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>; Julien Grall
> <julien.grall@xxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Ian
> Jackson <Ian.Jackson@xxxxxxxxxx>
> Subject: RE: [PATCH v6 13/14] x86: add iommu_ops to modify and flush
> IOMMU mappings
> 
> > 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...
> 

At the moment that is true. I guess Jan doesn't want to introduce another way 
for a guest to cause Xen to consume large amounts of memory.

  Paul

> Thanks
> Kevin
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