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MPC5775B/E, Problem with D-Cache and asynchronous MC Exceptions from MPU

Question asked by Peter Vranken on Jan 3, 2020
Latest reply on Jan 27, 2020 by Peter Vranken

Dear NXP Team,

 

Migrating an OS kernel from MPC5643L to MPC5775B/E, we face some issues
with D-cache and MPU. With respect to the kernel, these two are the major
difference between the two MCUs.

 

The core problem is the delayed MC exception from the MPU in case of a
forbidden store:

 

The MPU sits behind the cache and only protects the contents of the main
memory. The cache contents are however damaged by the store. For normal
process RAM there is a solution: The IVOR #1 handler reads the target
address of the failing store from the MPU and issues a cache block
invalidation instruction (dcbi, icbi) at that address. This makes the
temporarily bad memory contents be replaced by the still correct,
undamaged main memory contents on next use.

 

By appropriate synchronization instructions, we can ensure that all of
this repair is safely completed prior to the next context switch, i.e.
another process may have damaged data for a short while but it won't
become active during this critical period of time.

 

Situation is worse for OS data. Like in most code samples, we hold e.g.
the External Interrupt vector table in RAM. If an External Interrupt
appears after a process has issued a forbidden store into the table but
before the MPU has raised the MC exception, thus before the IVOR #4
handler could repair the cache contents, then the read and used interrupt
address would be wrong and the code would crash.

 

We have configured the cache as write-through for all of the RAM.

 

Following (undesired) solutions are known:

 

* We can put all sensitive OS data into an MMU protected memory section.
  MMU exceptions are raised synchronously and don't cause any headache.
  Drawback: The MMU can only control memory chunks of power-of-two size.
  We would loose a significant amount of RAM because of the alignment of
  subsequent data sections
* We can turn off the store buffers to minimize the delay of the MC
  exception from the MPU. (However, it is unclear if this would be enough
  without even setting the guard bit.) Drawback: Significant performance
  loss
 
Would you know more solutions? In particular, is there a possibility to
hinder the D cache from caching written data according to this idea: A
store at a cached address wouldn't change the cache block contents
immediately but invalidate the block so that subsequent reads would force
a cache line fill from the (protected, undamaged) main memory instead of
returning the illicitly overwritten cache block contents?

 

Kind regards

 

Peter

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