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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 21 Jul 2022 11:06:42 -0700
Message-ID: <CAHk-=whLGENEkjME_v_3P1SwhwjzH4GK2RLA3fn=yQNuyKLBkg@mail.gmail.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Joao Moreira <joao@overdrivepizza.com>,
        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>,
        "Cooper, Andrew" <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>,
        "Moreira, Joao" <joao.moreira@intel.com>,
        "Nuzman, Joseph" <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Gross, Jurgen" <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Peter Collingbourne <pcc@google.com>,
        Kees Cook <keescook@chromium.org>
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On Thu, Jul 21, 2022 at 10:56 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> this seems to work, let me go hack the kernel..

Am I missing something?

Isn't this generating

        movl $~IMM,%r10d
        negl %r10d
        cmpl %r10d,-4(%calldest)

for the sequence?

That seems bogus for two reasons:

 (a) 'neg' is not the opposite of '~'. Did you mean 'notl' or did you mean '-'?

     Or am I missing something entirely?

 (b) since you have that r10 use anyway, why can't you just generate the simpler

        movl $-IMM,%r10d
        addl -4(%calldest),%r10d

     instead? You only need ZF anyway.

     Maybe you need to add some "r10 is clobbered" thing, I don't know.

But again: I don't know llvm, so the above is basically me just doing
the "pattern matching monkey" thing.

             Linus