STM32H7Sx - Passing volatile data to lower HDPL levels

[Eliasvan]

On a secured STM32H7Sx device (in CLOSED or LOCKED product state), what is the recommended way of passing volatile data to lower HDPL levels?

According to the reference manual, the SBS_RSSCMDR register can be used to pass volatile data from HDPL 1, 2 or 3 to HDPL 0 (RSS), so that part of the answer is covered:

Bits 15:0 RSSCMD[15:0]: RSS command
The application can use this bitfield to pass on a command to the RSS, executed at the
next reset.

But what if you want to pass volatile data from HDPL 2 or 3 to HDPL 1?
(E.g., HDPL 2 wants to call into HDPL 1 so HDPL 2 first needs to write some volatile command somewhere, then execute a reset, then HDPL 0 (RSS) boots, then HDPL 1 boots and picks up the volatile command from somewhere.)
SBS_RSSCMDR cannot be used because RSS will interpret this value as well, and RSS might clear this value at the moment it boots HDPL 1.

[Jocelyn RICARD]

Hello @Eliasvan ,

this mechanism is not provided to be used in normal application as far as I know. It is used to launch a RSS command.

You can use specific location of RAM to transmit information from application to secure boot.

Secure boot have to avoid initialization of this specific location.

Best regards

Jocelyn

[Eliasvan]

Dear Jocelyn,

Thank you for your answer!

In order for me to fully accept your answer, I would need to know what location of RAM I can use without RSS wiping it, can you provide me this?

Thanks again!