Re: [RFC PATCH v2 05/15] xen/arm: pull in Linux atomics helpers and dependencies

[Ash Wilding <ash.j.wilding@xxxxxxxxx>]

Hey Jan,


>>
>> Note that Linux's arm32 atomics helpers use the READ_ONCE() and
>> WRITE_ONCE() macros defined in <asm-generic/rwonce.h>, while Linux's
>> arm64 atomics helpers use __READ_ONCE() and __WRITE_ONCE().
>
> And our ACCESS_ONCE() can't be used, or be made usable? I don't think
> we want a 3rd variant when we're already in the process of discussing
> how to fold the two ones we have right now.

Many thanks for the pointer, I'm still familiarising myself with Xen's
codebase and wasn't aware of ACCESS_ONCE(); yes, that's exactly what we
need which means we can drop Linux's <asm-generic/rwonce.h> completely.


That also means:

>
> I don't think weakening the checking is a good idea when the macros
> are being made available for general use. If they'd be renamed to be
> private flavors for use just in Arm's atomics, this would be a
> different thing.

This problem goes away, as does the need/desire to bump the minimum GCC
version up to 4.9 for xen/arm just to support the usage of C11 _Generic
in Linux's <linux/compiler_types.h>.

That said, agreed, I did think the way I'd done it was a tad suspect
hence the "possibly contentious" disclaimer :-) I'll keep this in mind
for similar porting work in future. 


>>
>> \ No newline at end of file
>
> This wants taking care of in any event - there are multiple instances
> in this patch (in fact it looks as if all of the new files had this
> issue), and I didn't check others.

Ack, will fix.


Thanks for taking the time to provide feedback!

Cheers,
Ash.