Re: [Xen-devel] Xen virtual IOMMU high level design doc

[Lan Tianyu <tianyu.lan@xxxxxxxxx>]

On 2016年11月24日 12:09, Edgar E. Iglesias wrote:
>>>> Hi,
>>>> > > >
>>>> > > > I have a few questions.
>>>> > > >
>>>> > > > If I understand correctly, you'll be emulating an Intel IOMMU in Xen.
>>>> > > > So guests will essentially create intel iommu style page-tables.
>>>> > > >
>>>> > > > If we were to use this on Xen/ARM, we would likely be modelling an 
>>>> > > > ARM
>>>> > > > SMMU as a vIOMMU. Since Xen on ARM does not use QEMU for emulation, 
>>>> > > > the
>>>> > > > hypervisor OPs for QEMUs xen dummy IOMMU queries would not really be 
>>>> > > > used.
>>>> > > > Do I understand this correctly?
>>> > > 
>>> > > I think they could be called from the toolstack. This is why I was
>>> > > saying in the other thread that the hypercalls should be general enough
>>> > > that QEMU is not the only caller.
>>> > > 
>>> > > For PVH and ARM guests, the toolstack should be able to setup the vIOMMU
>>> > > on behalf of the guest without QEMU intervention.
> OK, I see. Or, I think I understand, not sure :-)
> 
> In QEMU when someone changes mappings in an IOMMU there will be a notifier
> to tell caches upstream that mappings have changed. I think we will need to
> prepare for that. I.e when TCG CPUs sit behind an IOMMU.

For Xen side, we may notify pIOMMU driver about mapping change via
calling pIOMMU driver's API in vIOMMU.

> 
> Another area that may need change is that on ARM we need the map-query to 
> return
> the memory attributes for the given mapping. Today QEMU or any emulator 
> doesn't use it much but in the future things may change.
> 
> For SVM, whe will also need to deal with page-table faults by the IOMMU.
> So I think there will need to be a channel from Xen to Guesrt to report these.

Yes, vIOMMU should forward the page-fault event to guest. For VTD side,
we will trigger VTD's interrupt to notify guest about the event.

> 
> For example, what happens when a guest assigned DMA unit page-faults?
> Xen needs to know how to forward this fault back to guest for fixup and the
> guest needs to be able to fix it and tell the device that it's OK to contine.
> E.g PCI PRI or similar.
> 
> 


-- 
Best regards
Tianyu Lan

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