linux-kernel - Re: [PATCH v2 37/59] x86/putuser: Provide room for padding

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Date:   Mon, 5 Sep 2022 13:26:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 37/59] x86/putuser: Provide room for padding

On Mon, Sep 05, 2022 at 09:16:43AM +0200, Peter Zijlstra wrote:
> On Sat, Sep 03, 2022 at 10:26:45AM -0700, Linus Torvalds wrote:
> > On Fri, Sep 2, 2022 at 2:46 PM Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> > >
> > > Hmm. It doesn't look too horrible, but yeah, fi it still ends up
> > > getting the same padding overhead I'm not sure it ends up really
> > > mattering.
> > 
> > Oh, and I think it's buggy anyway.
> > 
> > If there are any tail-calls to a leaf function, the tracking now gets
> > out of whack. So it's no longer a "don't bother counting the last
> > level", now it ends up being a "count got off by one".
> 
> See validate_sibling_call(), it too calls validate_call().
> 
> (Although prehaps I should go s/sibling/tail/ on all that for clarity)

Ah, no, you're right and I needed more wake-up juice.