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Date:   Thu, 1 Nov 2018 09:56:34 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
Cc:     Linux-Kernel <linux-kernel@vger.kernel.org>, mingo@redhat.com,
        konrad.wilk@oracle.com, dwmw@amazon.co.uk, tglx@linutronix.de,
        Srinivas REDDY Eeda <srinivas.eeda@oracle.com>, bp@suse.de,
        hpa@zytor.com
Subject: Re: [PATCH 3/3] kprobes/x86: Simplify indirect-jump check in
 retpoline
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On Thu, Nov 01, 2018 at 10:02:14AM +0800, Zhenzhong Duan wrote:
> > Hmm, what about the case where we have RETPOLINE runtime disabled? Then
> > the CALL_NOSPEC alternative patches in an indirect call again, and the
> > retpolines are gone.
> 
> Is RETPOLINE runtime toggle supported in upstream? I don't see such code.

arch/x86/kernel/cpu/bugs.c look for the "nospectre_v2" and related
options. That will avoid X86_FEATURE_RETPOLINE from being set, and thus
the JMP_NOSPEC and CALL_NOSPEC alternatives will not patch out the
indirect jump / call.