Re: [XEN PATCH][for-4.19 1/9] xen/include: add macro LOWEST_POW2

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 6 Oct 2023, Julien Grall wrote:
> Hi Nicola,
> 
> On 06/10/2023 11:34, Nicola Vetrini wrote:
> > On 06/10/2023 12:22, Julien Grall wrote:
> > > On 06/10/2023 11:02, Nicola Vetrini wrote:
> > > > On 06/10/2023 11:29, Julien Grall wrote:
> > > > > Hi,
> > > > > 
> > > > > On 06/10/2023 09:26, Nicola Vetrini wrote:
> > > > > > The purpose of this macro is to encapsulate the well-known
> > > > > > expression
> > > > > > 'x & -x', that in 2's complement architectures on unsigned integers
> > > > > > will
> > > > > > give 2^ffs(x), where ffs(x) is the position of the lowest set bit in
> > > > > > x.
> > > > > > 
> > > > > > A deviation for ECLAIR is also introduced.
> > > > > 
> > > > > Can you explain why this is a deviation in ECLAIR rather than one with
> > > > > /* SAF-* */ (or whichever name we decide to rename to)? Is this
> > > > > because the code is correct from MISRA perspective but the tool is
> > > > > confused?
> > > > > 
> > > > 
> > > > The code does violate Rule 10.1 (a unary minus applied to an unsigned
> > > > value is
> > > > deemed inappropriate by MISRA), but rather than changing a whole lot of
> > > > places where this
> > > > construct is used (mainly in x86 code), the reasoning is that it makes
> > > > more sense to isolate
> > > > it and justify its presence by the fact that on 2's complement
> > > > architectures the result is
> > > > indeed correct.
> > > 
> > > This is explaining to me why you are adding LOWEST_POW2(). But this
> > > doesn't explain why you are not using /* SAF-* */ on top of
> > > LOWEST_POW2().
> > > 
> > > To me, we should only use ECLAIR specific deviation when this is a
> > > shortcoming with the tool.
> > > 
> > > Cheers,
> > 
> > Because of the way ECLAIR deviation comments work implies that in most cases
> > the real
> > place where to put the deviation is the usage site
> > (the so-called top expansion location of the macro). Now, for widely-used
> > macros this is
> > cumbersome and would clutter the code unnecessarily. It's way cleaner imo to
> > have a single
> > line in the configuration with a clear justification that is present in the
> > textual output
> > of the tool.
> 
> Just to clarify, you are saying that the following would not work for Eclair:
> 
> /* SAF-XXX */
> #define LOWEST_POW2()
> 
> Instead you would need the following:
> 
> void foo()
> {
>       /* SAF-XXX */
>       LOWEST()
> }
> 
> Am I correct? If so, would something like below (untested) work?
> 
> #define LOWEST_POW2(...) ({ \
>    /* SAFE-XXX */           \
>    ...
>    })
> 
> > But then there are tool interoperability considerations, that would call for
> > standardized
> > deviation mechanisms, if they do detect this as a violation (which I don't
> > know).
> 
> I don't think we need to know whether a tool detects it. We only need to know
> whether this is  violation to MISRA. If this is one, then this is a call to
> have a marker in the code.
> 
> If this is a false positive, then adding the deviation in the tool
> configuration is best (unless there are multiple tools affected).
> 
> > 
> > In the end, it could be done with a textual deviation, if that's preferred,
> > but keep in mind
> > that those are more fragile w.r.t. code movement.
> 
> If the comment is around the macro there are limited chance that this will be
> missed. But if you are worried about code movement, you should be worried
> about macro renaming with your approach (one may not know Eclair has a
> deviation) and/or function with the same name.
> 
> I am curious to know what the other thinks.

I agree.

I think that we should use the SAF-x-safe framework as much as possible.
That is the most flexible and easier to maintain deviation system we
have. If we can make it work in this specific case with Julien's
suggestion above, then great.

But it is becoming clear that the SAF-x-safe framework has limitations,
for instance  https://marc.info/?l=xen-devel&m=169657904027210

There are going to be cases where SAF-x-safe won't work. In those cases,
we will probably end up using an ECLAIR specific configuration. Those
cases should be hopefully few and should be well documented, also
outside of the ECLAIR config file, which is very ECLAIR specific and
optimized to be machine readable. 

We need another RST document under docs/misra to document any deviations
that are not dealt by SAF-x-safe comments. Today we are basically using
the notes section in the docs/misra/rules.rst table but that doesn't
scale.

So I think we should:
- create new RST file like docs/misra/deviations.rst
- deviations.rst will list any deviation that is not a SAF-x-safe
  deviation
- all ECLAIR special deviations in the ECLAIR config file should be
  documented in deviations.rst
- in the future special ECLAIR deviations in the config file should
  come with a new documentation entry in deviations.rst


This doesn't entirely address Julien's valid concern but at least it
makes it easier to recognize the problem when it occurs.