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Date:   Mon, 18 Jul 2022 23:43:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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On Mon, Jul 18, 2022 at 02:01:43PM -0700, Linus Torvalds wrote:
> On Mon, Jul 18, 2022 at 1:44 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > Yes, Peter and I came from avoiding a new compiler and the overhead for
> > everyone when putting the padding into the code. We realized only when
> > staring at the perf data that this padding in front of the function
> > might be an acceptable solution. I did some more tests today on different
> > machines with mitigations=off with kernels compiled with and without
> > that padding. I couldn't find a single test case where the result was
> > outside of the usual noise. But then my tests are definitely incomplete.
> 
> Well, it sounds like it most definitely isn't a huge and obvious problem.
> 
> > Yes, I know. But it was horrible enough to find the right spot in that
> > gcc maze. Then I was happy that I figured how to add the boolean
> > option. I let real compiler people take care of the rest. HJL???
> >
> > And we need input from the Clang folks because their CFI work also puts
> > stuff in front of the function entry, which nicely collides.
> 
> Yeah, looking at the gcc sources (I have them locally because it helps
> with the gcc bug reports I've done over the years), that
> ASM_OUTPUT_FUNCTION_PREFIX is very convenient, but it's too late to do
> any inter-function alignment for, because it's already after the usual
> function-alignment output.
> 
> So I guess the padding thing is largely tied together with alignment
> of the function start, so that idea of having different padding and
> alignment bytes doesn't workl that well.
> 
> At least not in that ASM_OUTPUT_FUNCTION_PREFIX model, which is how
> the gcc patch ends up being so small.

FWIW, when I was poking at this last week, I found that -falign-function
only seems to apply to the normal .text section and not to random other
sections with text we create.

Or rather, I was seeind a lot of unaligned functions that all had custom
sections despite explicitly using the (what I thought was a global)
function alignment toggle.