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Subject: Re: [PATCH v3 0/6] Static calls
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        Nadav Amit <namit@vmware.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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 <CAHk-=wiP4Z5r6tdkuD2DYocxG0zx=Kh+Z2k_LQ4QFvehe1+9uw@mail.gmail.com>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <faadced2-9343-8bbc-5bd6-8682382e7322@zytor.com>
Date:   Thu, 10 Jan 2019 12:51:53 -0800
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On 1/10/19 9:31 AM, Linus Torvalds wrote:
> On Wed, Jan 9, 2019 at 2:59 PM Josh Poimboeuf <jpoimboe@redhat.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.
> 

As far as I understand, on P6+ (and possibly earlier, but I don't know) it is
atomic on a 16-byte fetch datum, at least for Intel CPUs.

However, single byte accesses are always going to be safe.

	-hpa