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Date:   Thu, 21 Jul 2022 17:16:14 -0700
From:   Sami Tolvanen <samitolvanen@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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On Thu, Jul 21, 2022 at 08:27:12PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 21, 2022 at 11:06:42AM -0700, Linus Torvalds wrote:
> > 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?
> 
> No, you're right, I'm being daft again.
> 
> >  (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.
> 
> Right, lemme see if I can wrangle llvm to generate that.

That looks good to me. I updated my LLVM tree to generate this code
for the checks:

https://github.com/samitolvanen/llvm-project/commits/kcfi

Sami