[PATCH v1 2/5] cpukit: Add Exception Manager

[Kinsey Moore]

On 8/30/2021 07:50, Sebastian Huber wrote:
> On 30/08/2021 14:27, Kinsey Moore wrote:
>> On 8/30/2021 00:42, Sebastian Huber wrote:
>>> Hello Kinsey,
>>>
>>> why can't you use the existing fatal error extension for this? You 
>>> just have to test for an RTEMS_FATAL_SOURCE_EXTENSION source.  The 
>>> fatal code is a pointer to the exception frame.
>>
>> Unfortunately, the fatal error extensions framework necessarily 
>> assumes that the exception is fatal and so does not include the 
>> machinery to perform a thread dispatch or restore the exception frame 
>> for additional execution. It could theoretically be done in the fatal 
>> error extensions context, but it would end up being reimplemented for 
>> every architecture and you'd have to unwind the stack manually. I'm 
>> sure there are other ragged edges that would have to be smoothed over 
>> as well.
>
> Non-interrupt exceptions are not uniformly handled across 
> architectures in RTEMS currently. Adding the 
> RTEMS_FATAL_SOURCE_EXTENSION fatal source was an attempt to do this. I 
> am not that fond of adding a second approach unless there are strong 
> technical reasons to do this.
This was in an effort to formalize how recoverable exceptions are 
handled. Currently, it's done on on SPARC by handling exception traps as 
you would an interrupt trap since they share a common architecture on 
that platform. This representation varies quite a bit among platforms, 
so we needed a different mechanism.
>
> The initial fatal extensions are quite robust, you only need a stack, 
> valid read-only data and a valid code. So, using a user extension is 
> the right thing to do, but I don't thing we need a new one.
>
> Doing the non-interrupt exception processing on the stack which caused 
> the exception is a bit problematic, since the stack pointer might be 
> corrupt as well. It is more robust to switch to for example the 
> interrupt stack. If the exception was caused by an interrupt, then 
> this exception is not recoverable.

The non-interrupt exception processing occurs on the interrupt stack, 
not the thread/user stack. In the AArch64 support code provided, the 
stack is switched back to the thread/user stack before thread dispatch 
and exception frame restoration occurs.


>
> If the non-interrupt exception was caused by a thread, then you could 
> do some high level actions for some exceptions, such as floating-point 
> exceptions and arithmetic exceptions. If you get a data abort or 
> instruction error, then it is probably better to terminate the system.
I leave that decision to the handlers defined on this framework. In the 
case of the exception-to-signal mapping, I'm carrying over the existing 
exception set from the SPARC implementation.
>
> Non-interrupt exception handling is always architecture-dependent. It 
> is just a matter how you organize it. In general, the most sensible 
> way to deal with non-interrupt exceptions is to log the error somehow 
> and terminate the system. The mapping to signals is a bit of a special 
> case if you ask me. My preferred way to handle non-interrupt 
> exceptions would be to
>
> 1. switch to a dedicated stack
>
> 2. save the complete register set to the CPU exception frame
>
> 3. call the fatal error extensions with RTEMS_FATAL_SOURCE_EXTENSION 
> and the CPU exception frame (with interrupts disabled)
>
> Add a new API to query/alter the CPU exception frame, switch to the 
> stack indicated by the CPU exception frame, and restore the context 
> stored in the CPU exception frame. With these architecture-dependent 
> CPU exception frame support it should be possible to implement a high 
> level mapper to signals.
>
What you've described is basically what is happening here (the dedicated 
stack is currently the interrupt/exception stack on AArch64), but the 
low level details are necessarily contained within the CPU port in patch 
3/5. Support for this framework is not required for any CPU port, but 
CPU ports that do support it repurpose the existing code underlying the 
fatal error extensions with the additional support you described above. 
This does not exist in parallel to the fatal error extensions, but 
rather the fatal error extensions are moved on top of the Exception 
Manager for CPU ports that support it. The Exception Manager returns 
whether the exception was handled and the CPU port then calls the fatal 
error extensions if the exception wasn't handled. With this patch set, 
only an accessor was added to get the exception class, but my initial 
thoughts included manipulation of the execution address and several 
other more generic manipulators.


Kinsey