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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.09.18 at 08:41, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Paul Durrant
>> Sent: Wednesday, September 12, 2018 4:02 PM
>> 
>> > 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...

Hmm, good point. I keep forgetting that we allow guests to fiddle with
their own p2m.

Jan



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