Re: xen/arm: Hiding SMMUs from Dom0 when using ACPI on Xen

[Manish Jaggi <mjaggi@caviumnetworks.com>]

Hi Julien,

On 5/18/2017 8:27 PM, Julien Grall wrote:
> Hello,
>
> On 18/05/17 12:59, Manish Jaggi wrote:
>> On 2/27/2017 11:42 PM, Julien Grall wrote:
>>> On 02/27/2017 04:58 PM, Shanker Donthineni wrote:
>>>> Hi Julien,
>>>
>>> Hi Shanker,
>>>
>>> Please don't drop people in CC. In my case, any e-mail I am not CCed
>>> are skipping my inbox and I may not read them for a while.
>>>
>>>>
>>>> On 02/27/2017 08:12 AM, Julien Grall wrote:
>>>>>
>>>>>
>>>>> On 27/02/17 13:23, Vijay Kilari wrote:
>>>>>> Hi Julien,
>>>>>
>>>>> Hello Vijay,
>>>>>
>>>>>> On Wed, Feb 22, 2017 at 7:40 PM, Julien Grall <julien.grall@arm.com>
>>>>>> wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> There was few discussions recently about hiding SMMUs from DOM0 
>>>>>>> when
>>>>>>> using
>>>>>>> ACPI. I thought it would be good to have a separate thread for 
>>>>>>> this.
>>>>>>>
>>>>>>> When using ACPI, the SMMUs will be described in the IO Remapping
>>>>>>> Table
>>>>>>> (IORT). The specification can be found on the ARM website [1].
>>>>>>>
>>>>>>> For a brief summary, the IORT can be used to discover the SMMUs
>>>>>>> present on
>>>>>>> the platform and find for a given device the ID to configure
>>>>>>> components such
>>>>>>> as ITS (DeviceID) and SMMU (StreamID).
>>>>>>>
>>>>>>> The appendix A in the specification gives an example how 
>>>>>>> DeviceID and
>>>>>>> StreamID can be found. For instance, when a PCI device is both
>>>>>>> protected by
>>>>>>> an SMMU and MSI-capable the following translation will happen:
>>>>>>>         RID -> StreamID -> DeviceID
>>>>>>>
>>>>>>> Currently, SMMUs are hidden from DOM0 because they are been used by
>>>>>>> Xen and
>>>>>>> we don't support stage-1 SMMU. If we pass the IORT as it is, DOM0
>>>>>>> will try
>>>>>>> to initialize SMMU and crash.
>>>>>>>
>>>>>>> I first thought about using a Xen specific way (STAO) or 
>>>>>>> extending a
>>>>>>> flag in
>>>>>>> IORT. But that is not ideal.
>>>>>>>
>>>>>>> So we would have to rewrite the IORT for DOM0. Given that a 
>>>>>>> range of
>>>>>>> RID can
>>>>>>> mapped to multiple ranges of DeviceID,
Do you envisage a scenario where same RID can map to multiple StreamIDs 
belonging to different SMMUs ?
>>>>>>> we would have to translate
>>>>>>> RID one by
>>>>>>> one to find the associated DeviceID. I think this may end up to
>>>>>>> complex code
>>>>>>> and have a big IORT table.
>>>>>>
>>>>>> Why can't we replace Output base of IORT of PCI node with SMMU 
>>>>>> output
>>>>>> base?.
>>>>>> I mean similar to PCI node without SMMU, why can't replace output 
>>>>>> base
>>>>>> of PCI node with
>>>>>> SMMU's output base?.
>>>>>
>>>>> Because I don't see anything in the spec preventing one RC ID mapping
>>>>> to produce multiple SMMU ID mapping. So which output base would you
>>>>> use?
>>>>>
>>>>
>>>> Basically, remove SMMU nodes, and replaces output of the PCIe and 
>>>> named
>>>> nodes ID mappings with ITS nodes.
>>>>
>>>> RID --> StreamID  --> dviceID  --> ITS device id = RID --> dviceID  
>>>> -->
>>>> ITS device id
>>>
>>> Can you detail it? You seem to assume that one RC ID mapping range
>>> will only produce ID mapping range. AFAICT, this is not mandated by
>>> the spec.
>>>
>> You are correct that it is not mandated by the spec, but AFAIK there
>> seems to be no valid use case for that.
>
> Xen has to be compliant with the spec, if the spec says something then 
> we should do it unless there is a strong reason not to.
>
> In this case, it is not too difficult to implement the suggestion I 
> wrote a couple of months ago. So why would we try to put us in a corner?
>
See below
>>
>> RID range should not overlap between ID Array entries.
>
> I believe you misunderstood my point here. So let me give an example. 
> My understanding of the spec is it is possible to have:
>
> RC A
>  // doesn't use SMMU 0 so just outputs DeviceIDs to ITS GROUP 0
>  // Input ID --> Output reference: Output ID
> 0x0000-0xffff --> ITS GROUP 0 : 0x0000->0xffff
>
> SMMU 0
> // Note that range of StreamIDs that map to DeviceIDs excludes
> // the NIC 0 DeviceID as it does not generate MSIs
>  // Input ID --> Output reference: Output ID
> 0x0000-0x01ff --> ITS GROUP 0 : 0x10000->0x101ff
> 0x0200-0xffff --> ITS GROUP 0 : 0x20000->0x207ff
>
> // SMMU 0 Control interrupt is MSI based
>  // Input ID --> Output reference: Output ID
> N/A --> ITS GROUP 0 : 0x200001
>
I could have misunderstood so I am stating my understanding so far .. 
please feel free to correct me :)

In the IORT table using the  PCI-RC node, SMMU node and ITS node, 
RID->StreamID->Device-ID  mapping can be generated.
As per IORT spec toady, same RID can be mapped to different StreamIDs 
using two ID Array elements with same RID range but different output 
reference.
There exists no use case for such a scenario hence a clarification is 
required in IORT spec which states that RID range cannot overlap in the 
ID array.

with this clarification in place, it is straight-forward to map RID to a 
device-ID by replacing output of SMMU to output of RCI-RC

>> I believe this would be updated in the next IORT spec revision.
>
> Well, Xen should still support current revision of IORT even if the 
> next version add more restriction.
>
> Cheers,
>

-Manish


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