Re: [Xen-devel] [PATCH v2 0/2] ARM: ACPI: IORT: Hide SMMU from hardware domain's IORT table

[Julien Grall <julien.grall@xxxxxxx>]

Hello,

On 25/09/17 05:22, Manish Jaggi wrote:
> On 9/22/2017 7:42 PM, Andre Przywara wrote:
> > Hi Manish,
> >
> > On 11/09/17 22:33, mjaggi@xxxxxxxxxxxxxxxxxx wrote:
> > > From: Manish Jaggi <mjaggi@xxxxxxxxxx>
> > >
> > > The set is divided into two patches. First one calculates the size of 
> > > IORT
> > > while second one writes the IORT table itself.
> > It would be good if you could give a quick introduction *why* this set
> > is needed here (and introduce IORT to the casual reader).
> > In general some more high-level documentation on your functions would be
> > good, as it took me quite some time to understand what each function 
> > does.
> ok, will add more documentation.
> > So my understanding is:
> > phase 1:
> > - go over each entry in each RC node
> Rather than each entry (which could be a large number) I am taking the 
> complete range and checking it with the same logic.
> If the ID range is a subset or a super-set of id range in smmu, new id 
> range is created.
> So if pci_rc node has an id map {p_input-base,p_output-base,p_out_ref, 
> p_count} and it an output reference to smmu node with id-map
> {s_input-base, s_output-base,s_out_ref,  s_count}, based on the the the 
> s_count and s_input/p_output the new id-map is created with {p_input, 
> s_output, s_out_ref, adjusted_count}
> update_id_mapping function does that.
>
> So I am following the same logic. We can chat over IRC / I can give a 
> code walk-through ...
> > -   if that points to an SMMU node, go over each outgoing ITS entry and
> > find overlaps with this RC entry
> > -     for each overlap create a new entry in a list with this RC
> > pointing to the ITS directly
> >
> > phase 2, creating the new IORT
> > - go over each RC node
> > -   if that points to an ITS, copy through IORT entries
> > -   if that points to an SMMU, replace with the remapped entries
> > - go over each ITS node
> > -   copy through IORT entries
> Thats exactly what this patch does.
> > So I believe this would do the trick and you end up with an efficient
> > representation of the IORT without SMMUs - at least for RC nodes.
> >
> > After some brainstorming with Julien we found two problems:
> > 1) This only covers RC nodes, but not "named components" (platform
> > devices), which we will need. That should be fixable by removing the
> > hardcoded IORT node types in the code and treating NC nodes like RC 
> > nodes.
> Yes, so first we can take this as a base, once this is ok, I can add 
> support for named components.
> > 2) Eventually we will need *virtual* deviceID support, for DomUs. Now we
I am a bit surprised that you answered to the e-mail but didn't provide 
any opinion on 2).
> > could start introducing that already, also doing some virtual mapping
> > for Dom0. The ITS code would then translate each virtual device ID that
> > Dom0 requests into a hardware device ID.
> > I agree that this means a lot more work, but we will need it anyway.
I am a bit surprised that you answered to the e-mail but didn't provide 
any opinion on 2).
> > I think 1) can be solved using this series as a base. I have quite some
> > comments ready for the patches, shall we follow this route.
> >
> > 2) obviously would change the game completely. We need to sit down and
> > design this properly. Probably this means that Xen parses the IORT and
> > builds internal representations of the mappings, which are consulted as
> > needed when passing through devices. The guest's (that would include
> > Dom0) IORT would then be generated completely from scratch.
> >
> > I would like to hear your opinion on this. I will try to discuss the
> > feasibility of 2) with people at Connect. It would be good if we could
> > decide whether this is the way to go or we should use a solution based
> > on this series.
Andre, Stefano and I had a chat during connect about how we want to see 
IORT support in Xen. In the near future, IORT will be used for different 
components accross the hypervisor (ITS, SMMU...) and as a way to
communicate the topology to the guests.

The IORT for the hardware domain is just a specific case as it is based 
pre-existing information. But because of removing nodes (e.g SMMU nodes 
and probably the PMU nodes), it is basically a full re-write.
So I would consider of full separate the logic of generating the IORT 
table from the host IORT table. By that I mean not browsing the host 
IORT when generating the host.
This has two benefits:
	1) The code generation could be re-used for generating the guest IORT 
later on.
        2) See 2) from Andre
	3) We could decide in a finer grain which devices (e.g platform device) 
Dom0 can see.
So, IHMO, we should take a different approach from this series and 
extending the scope of it. Rather than focusing on only IORT for the 
hardware, I would be better to see IORT as a whole. I.e  how IORT will 
interact with the hypervisor?
For instance, likely you would need to parse the IORT in quite a few 
places in Xen. It would be better to get IORT parsed only once and store 
the information in Xen like data-structures.
This require more work than this current scope of this series. But I 
think it would helful for future work such as PCI passthrough support.
Any opinions?

Cheers,

--
Julien Grall

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