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Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code
To: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
References: <20180110010652.404145126@linutronix.de>
 <20180110011350.855878109@linutronix.de>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <ce6c99be-b8ea-f789-ca7c-625c99b2a7a2@intel.com>
Date: Tue, 9 Jan 2018 18:02:53 -0800
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On 01/09/2018 05:06 PM, Thomas Gleixner wrote:
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -79,6 +79,7 @@ enum spectre_v2_mitigation_cmd {
>  	SPECTRE_V2_CMD_RETPOLINE,
>  	SPECTRE_V2_CMD_RETPOLINE_GENERIC,
>  	SPECTRE_V2_CMD_RETPOLINE_AMD,
> +	SPECTRE_V2_CMD_IBRS,
>  };

A few nits on this:

IBRS should not default on anywhere, which goes double when retpolines
are available.

I think I'd also prefer that we separate the IBRS and retpoline enabling
so that you can do both if you want.  They do nearly the same thing in
practice, but I can't convince myself that you never ever need IBRS once
retpolines are in place.