linux-kernel - Re: [PATCH v3 0/6] Static calls

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Date:   Thu, 10 Jan 2019 09:31:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 0/6] Static calls

On Wed, Jan 9, 2019 at 2:59 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> NOTE: At least experimentally, the call destination writes seem to be
> atomic with respect to instruction fetching.  On Nehalem I can easily
> trigger crashes when writing a call destination across cachelines while
> reading the instruction on other CPU; but I get no such crashes when
> respecting cacheline boundaries.

I still doubt ifetch is atomic on a cacheline boundary for the simple
reason that the bus between the IU and the L1 I$ is narrower in older
CPU's.

Also, the fill of the L1 I$ from the (cache coherent L2) may not be a
cacheline at a time either.

That said, the fetch may be sufficiently ordered that it works in
practice. It _would_ be absolutely lovely to be able to do things like
this.

I do agree with Nadav that if there's some way to avoid this, it would
be good. I'm not in general a huge fan of compiler plugins (compiler
instability is just about my worst fear, and I feel plugins tend to
open up that area a lot), but it does feel like this might be
something where compiler tweaking would possibly be the cleanest
approach.

             Linus