linux-kernel - Re: Official documentation from Intel stating that poking INT3 (single-byte) concurrently is OK ?

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Date:   Wed, 22 Feb 2023 10:20:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Olivier Dion <odion@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: Official documentation from Intel stating that poking INT3
 (single-byte) concurrently is OK ?

On Tue, Feb 21, 2023 at 01:42:58PM -0500, Mathieu Desnoyers wrote:
> On 2023-02-21 12:50, Steven Rostedt wrote:
> > On Tue, 21 Feb 2023 11:44:42 -0500
> > Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> > 
> > > Hi Peter,
> > > 
> > > I have emails from you dating from a few years back unofficially stating
> > > that it's OK to update the first byte of an instruction with a single-byte
> > > int3 concurrently:
> > > 
> > > https://lkml.indiana.edu/hypermail/linux/kernel/1001.1/01530.html
> > > 
> > > It is referred in the original implementation of text_poke_bp():
> > > commit fd4363fff3d9 ("x86: Introduce int3 (breakpoint)-based instruction patching")
> > > 
> > > Olivier Dion is working on the libpatch [1,2] project aiming to use this
> > > property for low-latency/low-overhead live code patching in user-space as
> > > well, but we cannot find an official statement from Intel that guarantees
> > > this breakpoint-bypass technique is indeed OK without stopping the world
> > > while patching.
> > > 
> > > Do you know where I could find an official statement of this guarantee ?
> > > 
> > 
> > The fact that we have been using it for over 10 years without issue should
> > be a good guarantee ;-)
> > 
> > I know you probably prefer an official statement, and I thought they
> > actually gave one, but can't seem to find it.
> 
> I recall an in-person discussion with Peter Anvin shortly after he got the
> official confirmation, but I cannot find any public trace of it. I suspect
> Intel may have documented this internally only.

My 2ct, ISTR this also having been vetted by AMD, perhaps they did
manage to write it down somewhere.