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

[Julien Grall <julien@xen.org>]

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/

> 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?

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,

-- 
Julien Grall