Re: [PATCH v5] x86: use builtins to read eflags

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Mar 17, 2022 at 4:25 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> > I still think that from a sanity standpoint, it would be good to
> > actually strengthen the semantics of "asm volatile" to literally act
> > as - and be ordered with - volatile memory accesses.
> >
> > But I guess that's water under the bridge.
>
> That is what it has actually done since forever.  See C 5.1.2.3.  For
> GCC, "asm volatile" has a side effect like in /2 there as well, as does
> unspec_volatile (an internal GCC thing used to implement certain
> builtins, among other things).

Oh, so two "asm volatile" statements _are_ in fact defined to be
ordered wrt each other?

Because the gcc docs certainly don't say that ;(

Yeah, yeah, dead code can be removed, whether volatile or not. That's
true of "*(volatile int *)x" too. It's not the dead code that is the
interesting case, though..

Is this also well-defined ordering-wise:

    asm volatile("do_something");
    WRITE_ONCE(x, 1);

(where "WRITE_ONCE()" is that kernel macro that just uses a volatile
pointer assignment to force the access)?

And could we get that documented?

            Linus