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Message-ID: <CAHk-=whLGENEkjME_v_3P1SwhwjzH4GK2RLA3fn=yQNuyKLBkg@mail.gmail.com>
Date: Thu, 21 Jul 2022 11:06:42 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Joao Moreira <joao@...rdrivepizza.com>,
LKML <linux-kernel@...r.kernel.org>,
"the arch/x86 maintainers" <x86@...nel.org>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
"Cooper, Andrew" <andrew.cooper3@...rix.com>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Johannes Wikner <kwikner@...z.ch>,
Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
"Moreira, Joao" <joao.moreira@...el.com>,
"Nuzman, Joseph" <joseph.nuzman@...el.com>,
Steven Rostedt <rostedt@...dmis.org>,
"Gross, Jurgen" <jgross@...e.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Peter Collingbourne <pcc@...gle.com>,
Kees Cook <keescook@...omium.org>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
On Thu, Jul 21, 2022 at 10:56 AM Peter Zijlstra <peterz@...radead.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
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