Message-ID: <786f4aed-2c30-806b-409b-23a60b3d7571@efficios.com>
Date:   Tue, 21 Feb 2023 11:44:42 -0500
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To:     "H . Peter Anvin" <hpa@zytor.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        rostedt <rostedt@goodmis.org>, Olivier Dion <odion@efficios.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Jiri Kosina <jkosina@suse.cz>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Official documentation from Intel stating that poking INT3
 (single-byte) concurrently is OK ?
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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 ?

Thanks!

Mathieu

[1] https://www.dorsal.polymtl.ca/files/may2022/odion_may2022_libpatch_binary_patcher.pdf
[2] https://git.sr.ht/~old/libpatch

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com