linux-kernel - Re: [PATCH] docs/memory-barriers.txt: volatile is not a barrier() substitute

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Date:   Tue, 1 Feb 2022 08:37:49 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, llvm@...ts.linux.dev,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Len Baker <len.baker@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] docs/memory-barriers.txt: volatile is not a barrier() substitute

On Tue, Feb 1, 2022 at 12:53 AM Kees Cook <keescook@...omium.org> wrote:

> > +
> > +According to `the GCC docs on inline asm
> > +https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile`_:
> > +
> > +  asm statements that have no output operands and asm goto statements,
> > +  are implicitly volatile.
>
> Does this mean "volatile" _is_ needed when there are operands, etc?

It depends on what you want to express. The idea here is to give a way to
gcc for optimizing out anything with an output, like x86 rdtsc() when the
result is not used, which is sensible. If there is no output, such as in
writel(), you don't need 'volatile' because gcc can assume that an
inline asm without outputs has side-effects already.

A case where you need to add volatile is for (void)readl(ADDR),
which is an operation that has an output as well as a side-effect.

          Arnd