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Re: [Xen-devel] [RFC PATCH 0/4] Add missing default labels to switch statements

On 2/23/19 1:34 AM, Julien Grall wrote:

On 22/02/2019 22:38, Stefano Stabellini wrote:
On Fri, 22 Feb 2019, Andrew Cooper wrote:
On 22/02/2019 22:11, Julien Grall wrote:
Hi Stefano,

On 22/02/2019 21:58, Stefano Stabellini wrote:
On Fri, 22 Feb 2019, Andrew Cooper wrote:
On 22/02/2019 21:00, Stefano Stabellini wrote:
On Fri, 22 Feb 2019, Julien Grall wrote:
BTW, I checked the series with -Wswitch-default:
Warn whenever a switch statement does not have a default case.
Furthermore, using BUG() is a pretty bad idea in switch.
It is and not only in the switch. The reason I put BUG is that I tried
to follow
the existing "error handling" at those places.
It is not because BUG() is been used today in some places that we need to
continue to spread it.

Use of BUG() itself is another topic which will also need to be
So we should not add more of them...
Again, I see this as a dedicated change. So, in the current series I think
it is
acceptable to use the existing way of error handling if any at all.
That's not how it works in upstream. If you know some constructs are wrong, it
is best to try to address partially the problem directly then having so you
reduce the amounts of change afterwards.

So please try to not introduce more BUG() in the code base.
Hi Oleksandr, Julien,

Julien's right that we should not introduce any more BUG()s. In fact,
each of them makes the code less safe, not more safe! The purpose of
MISRAC 16.4 is "defensive programming": write the code in a way that is
more (not less!) resilient to failure.

So, I think it is a good idea to introduce a default label because it
can help us spot unexpected issues. Instead of calling BUG() in the
default handler, which is detrimental, we should return an error when
possible, or just print a warning.
domain_crash() is almost always better than BUG().  It is very obvious
if it gets hit, and wont crash Xen.
That's a good suggestion.

As 16.4 clearly state, even a simple comment would be enough to address
the rule. We just need to explain why a default label is not needed.
Such as:

     /* unreachable because blah and blah */
What a simple comment doesn't do is avoid breaking -Wswitch.
I don't know how to reconcile 16.4 with -Wswitch. One could argue that
-Wswitch could be a good way to address 16.4, but then we introduce a
compiler specific requirement. Typically gcc is not the compiler of
choice for these environments, unfortunately forcing gcc is not an
Well, you could build with GCC and then build with your custom
compiler... But, GCC is pretty much the only choice for Xen on Arm today
as we don't build with clang and I pretty doubt we can build with compcert.
So the suggestion I had was to have an overall CONFIG_MISRA which we can
hide some of this nonsense behind, and then

#define MISRA_BLE_DEFAULT default:

So when you disable CONFIG_MISRA, your compiler starts being able to
help you again.
This is actually a good way to make progress which could make everybody
happy. (And it wouldn't require a slow back-and-forth with third parties
to ask difficult questions.)
I can tell you I am not happy with that :). We would make the code more
obscure. So it raises question on what would be the benefits of adopting
the rule in Xen.
And this is the very first change which leads to those CONFIG_MISRA
things, but what happens next when we want to add even more of those?
I do agree here that code will become just a huge amount of #ifdef's
at the end of the day.
But maybe the first question is how much we need to adhere to those
I am no way an expert in certification, but from my POV you either
comply or not. There is no solution in between. As per my understanding
MISRA allows some documented exceptions, but I'm just curious if they
expect most of the code to have those exceptions...
Experts, please step in...
  What are the consequences of not following them in Xen Project? I
know that some upstream project chose to not apply to all the rules.
Not all of the upstream projects want to be certified...

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