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Re: [Xen-devel] [PATCH v6 1/4] xen: introduce SYMBOL

On Tue, 15 Jan 2019, Jan Beulich wrote:
> > Yes, this instance is only the tip of the
> > iceberg, we have a long road ahead, but we shouldn't really give up
> > because it is going to be difficult :-) Stewart's approach would
> > actually be compliant and help toward reducing reliance on undefined
> > behavior.
> > 
> > Would you be OK if I rework the series to follow his approach using
> > intermediate variables? See the attached patch as a reference, it only
> > "converts" _start and _end as an example. Fortunately, it will be
> > textually similar to the previous SYMBOL returning unsigned long version
> > of the series.
> Well, I've given reasons why I dislike that, and why (I think) it was
> done without such intermediate variables. Nevertheless, if this is
> _the only way_ to achieve compliance, I don't think I could
> reasonably NAK it.
> The thing that I don't understand though is how the undefined
> behavior (if there really is any) goes away: Even if you compare
> the contents of the variables instead of the original (perhaps
> casted) pointers, in the end you still compare what C would
> consider pointers to different objects. It's merely a different
> way of hiding that fact from C.

I saw that Stewart wrote a long and detailed reply, but this is my short
take on this. I don't think so: with this approach there are no dubious
pointers in C land at all[1]. It is perfectly fine to have addresses as
integers in C, compare and subtracts addresses as integers, then casting
one of them to a pointer and accessing a structure with the pointer.
_start becomes only defined and used outside of C. I think both C and
MISRAC compliance would be satisfied.

([1]: There a catch with the way we use the pointers in alternative.c, both
x86 and arm, but is easy to fix in a follow-up series. Everything else
is taken care of.)

> Undefined behavior would imo
> go away only if those comparisons/subtractions didn't happen
> in C anymore. IOW - see my .startof.() / .sizeof.() proposal.
> > If you are OK with it, do you have any suggestions on how would you like
> > the intermediate variables to be called? I went with _start/start_ and
> > _end/end_ but I am open to suggestions. Also to which assembly file you
> > would like the new variables being added -- I created a new one for the
> > purpose named var.S in the attached example.
> First of all we should explore whether the variables could also be
> linker generated, in particular to avoid the current symbols to be
> global (thus making it impossible to access them from C files in the
> first place).

That would be fantastic. I looked around, I found interesting things
like PROVIDE, but I don't think what you describe is possible. The
linker scripts only define symbols, they cannot set or define variables.

> Failing that, I don't think it matters much where these
> helper symbols live, and hence your choice is probably fine (I'd
> prefer though if, just like on Arm, the x86 file didn't live in the
> boot/ subdirectory; in the end it might even be possible to have
> some of them in xen/common/var.S).

OK, I'll move the x86 var.S to xen/arch/x86/x86_64. I cannot share var.S
because arm32 is using long instead of quad.

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