[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |