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

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Message-ID: <CAHk-=wgHgSJ-suTT6V6_2USPDrdmQweLR_M_YHjb8OsQvYWn-w@mail.gmail.com>
Date:   Thu, 17 Mar 2022 16:31:19 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Bill Wendling <morbo@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, llvm@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH v5] x86: use builtins to read eflags

On Thu, Mar 17, 2022 at 4:25 PM Segher Boessenkool
<segher@...nel.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