linux-kernel - Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

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Date:   Tue, 9 Jan 2018 22:11:02 -0600
From:   Justin Forbes <jmforbes@...uxtx.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

On Tue, Jan 9, 2018 at 8:02 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 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.

Fairly strong agreement here. IBRS being separately configurable gives
us an option for the paranoid, and allows distros to ship with it off
by default.