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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        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>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, samitolvanen@google.com
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
In-Reply-To: <YtXOMPpmx8TcFtOX@worktop.programming.kicks-ass.net>
References: <20220716230344.239749011@linutronix.de> <87wncauslw.ffs@tglx>
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 <87o7xmup5t.ffs@tglx> <YtXOMPpmx8TcFtOX@worktop.programming.kicks-ass.net>
Date:   Tue, 19 Jul 2022 00:22:26 +0200
Message-ID: <87lesqukm5.ffs@tglx>
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On Mon, Jul 18 2022 at 23:18, Peter Zijlstra wrote:
> On Mon, Jul 18, 2022 at 10:44:14PM +0200, Thomas Gleixner wrote:
>> And we need input from the Clang folks because their CFI work also puts
>> stuff in front of the function entry, which nicely collides.
>
> Right, I need to go look at the latest kCFI patches, that sorta got
> side-tracked for working on all the retbleed muck :/
>
> Basically kCFI wants to preface every (indirect callable) function with:
>
> __cfi_\func:
> 	int3
>         movl $0x12345678, %rax
>         int3
>         int3
> \func:
>         endbr
> \func_direct:
>
> Ofc, we can still put the whole:
>
> 	sarq	$5, PER_CPU_VAR(__x86_call_depth);
> 	jmp	\func_direct
>
> thing in front of that. But it does somewhat destroy the version I had
> that only needs the 10 bytes padding for the sarq.

Right, because it needs the jump. I was just chatting with Jaoa about
that over IRC.

The jump slow things down. Jaoa has ideas and will reply soonish.

Thanks,

        tglx