linux-kernel - Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

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Message-ID: <20190108190104.GC1900@hirez.programming.kicks-ass.net>
Date:   Tue, 8 Jan 2019 20:01:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Edward Cree <ecree@...arflare.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        songliubraving@...com
Subject: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

On Tue, Jan 08, 2019 at 10:28:02AM -0800, Nadav Amit wrote:
> Is it really that important for debugging to get the instructions at the
> time of execution? Wouldn’t it be easier to annotate the instructions that
> might change? After all, it is not as if any instruction can change to any
> other instruction.

I think PT has a bitstream encoding of branch-taken; to decode and
follow the actual code-flow you then need to have the actual and
accurate branch target from the code. If we go muck about with the code
and change that, decoding gets somewhat 'tricky'.

Or something along those lines..