linux-kernel - Re: [PATCH -v5 00/17] Rewrite x86/ftrace to use text

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Date:   Mon, 11 Nov 2019 21:39:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jpoimboe@...hat.com, jeyu@...nel.org,
        bpf@...r.kernel.org, daniel@...earbox.net, davem@...emloft.net,
        kernel-team@...com
Subject: Re: [PATCH -v5 00/17] Rewrite x86/ftrace to use text_poke (and more)

On Mon, Nov 11, 2019 at 11:47:28AM -0800, Alexei Starovoitov wrote:

> One more question.
> What is the reason you stick to int3 style poking when 8 byte write is atomic?
> Can text_poke() patch nop5 by combining the call/jmp5 insn with extra 3 bytes
> after the nop and write 8 ?

I think that question came up a while back (in one of the many
static_call threads IIRC), and it basically boils down to there being
far too many x86 uarchs to be sure of anything.

Instruction fetch width is not always (well) specified and aligning
instructions on i-fetch boundaries (or ensuring they don't cross) was
deemed too fragile (also, it wastes space).

This scheme is blessed by the hardware folks, and while it might be
a little cumbersome, it isn't too horrible. Also, actually using that
exception turns out to be beneficial for tracing text changes, see also
this thread:

  https://lkml.kernel.org/r/20191025130000.13032-2-adrian.hunter@intel.com