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Date:   Thu, 21 Jul 2022 20:27:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Joao Moreira <joao@overdrivepizza.com>,
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        the arch/x86 maintainers <x86@kernel.org>,
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        Josh Poimboeuf <jpoimboe@kernel.org>,
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        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>,
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        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 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.

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

R11,R11 are caller-saved, and since this is the actual call site, the
caller must already have saved them or marked them clobbered.