Re: [RFC PATCH v4 6/8] xen/arm: scmi: introduce SCI SCMI SMC multi-agent driver

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

On 18/06/2025 10:28, Julien Grall wrote:
> Hi Oleksii,
>
> On 19/05/2025 16:50, Oleksii Moisieiev wrote:
>> This patch introduces SCI driver to support for ARM EL3 Trusted 
>> Firmware-A
>> (TF-A) which provides SCMI interface with multi-agnet support, as shown
>
> s/multi-agnet/multi-agent/
>
will fix.
>> below.
>>
>>    +-----------------------------------------+
>>    |                                         |
>>    | EL3 TF-A SCMI                           |
>>    +-------+--+-------+--+-------+--+-------++
>>    |shmem0 |  |shmem1 |  |shmem2 |  |shmemX |
>>    +-----+-+  +---+---+  +--+----+  +---+---+
>> smc-id0 |        |         |           |
>> agent0  |        |         |           |
>>    +-----v--------+---------+-----------+----+
>>    |              |         |           |    |
>>    |              |         |           |    |
>>    +--------------+---------+-----------+----+
>>           smc-id1 |  smc-id2|    smc-idX|
>>           agent1  |  agent2 |    agentX |
>>                   |         |           |
>>              +----v---+  +--v-----+  +--v-----+
>>              |        |  |        |  |        |
>>              | Dom0   |  | Dom1   |  | DomX   |
>>              |        |  |        |  |        |
>>              |        |  |        |  |        |
>>              +--------+  +--------+  +--------+
>>
>> The EL3 SCMI multi-agent firmware expected to provide SCMI SMC/HVC 
>> shared
>> memory transport for every Agent in the system.
>>
>> The SCMI Agent transport channel defined by pair:
>>   - smc-id: SMC/HVC id used for Doorbell
>>   - shmem: shared memory for messages transfer, Xen page aligned,
>>   p2m_mmio_direct_nc.
>
> It is not clear why we nention Xen page aligned and 
> p2m_mmio_direct_nc. Is this multi-agent protocol tied to Xen?
>
Xen allows mapping only page aligned chunks between domains. Current 
implementation supports only page-aligned chunks.

This means that we support only one channel per page, where the shared 
memory starts at the beginning of the page.


> That said... p2m_mmio_direct_nc is a type used in the stage 2 
> page-tables to indicate how we restrict access from the domain.
>
> The resulting memory attribute will be a combination of stage-1 + 
> stage-2. In the future, we may decide to use FWB which will allow Xen 
> to force a specific memory attribute.
>
> This is also purely internal decision. In the documentation, you 
> should spell out the memory attribute that should be used. From the 
> discussion on this patch, it is still unclear whether the region 
> should be mapped as Device nGnRE or normal memory non-cacheabl.
>
> Cheers,
>
I will reword this to explicitly mention the correct memory attribute.

Documentation will be updated.