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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     David Laight <David.Laight@ACULAB.COM>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     "x86@kernel.org" <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.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>
Subject: RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation
In-Reply-To: <24673619e9e2411bb1d5f287aab2aa87@AcuMS.aculab.com>
References: <20220716230344.239749011@linutronix.de>
 <f9fd86acac4f49bc8f90b403978e9df3@AcuMS.aculab.com> <8735ezye00.ffs@tglx>
 <24673619e9e2411bb1d5f287aab2aa87@AcuMS.aculab.com>
Date:   Sun, 17 Jul 2022 21:15:44 +0200
Message-ID: <87wncbwnxb.ffs@tglx>
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On Sun, Jul 17 2022 at 17:56, David Laight wrote:
> From: Thomas Gleixner
>> On Sun, Jul 17 2022 at 09:45, David Laight wrote:
> I was thinking about what happens after the RSB has underflowed.
> Which is when (I presume) the BTB based speculation happens.
>
>> The intra function call in the retpoline is of course adding a RSB entry
>> which points to the speculation trap, but that gets popped immediately
>> after that by the return which goes to the called function.
>
> I'm remembering the 'active' instructions in a retpoline being 'push; ret'.
> Which is an RSB imbalance.

Looking at the code might help to remember correctly:

        call   1f
        speculation trap
1:      mov     %reg, %rsp
        ret

Thanks,

        tglx