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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 18 Jul 2022 12:51:19 -0700
Message-ID: <CAHk-=wjpzVRU0Yr_0DJSB_bKHW3_74UucNpJBjxfHPo_R=PYNg@mail.gmail.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     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>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
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On Mon, Jul 18, 2022 at 12:30 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Let the compiler add a 16 byte padding in front of each function entry
> point and put the call depth accounting there. That avoids calling out
> into the module area and reduces ITLB pressure.

Ooh.

I actually like this a lot better.

Could we just say "use this instead if you have SKL and care about the issue?"

I don't hate your module thunk trick, but this does seem *so* much
simpler, and if it performs better anyway, it really does seem like
the better approach.

And people and distros who care would have an easy time adding that
simple compiler patch instead.

I do think that for generality, the "-mforce-function-padding" option
should perhaps take as an argument how much padding (and how much
alignment) to force:

    -mforce-function-padding=5:16

would force 5 bytes of minimum padding, and align functions to 16
bytes. It should be easy to generate (no more complexity than your
current one) by just making the output do

        .skip 5,0xcc
        .palign 4,0xcc

and now you can specify that you only need X bytes of padding, for example.

                        Linus