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

From: Justin Forbes <jmforbes@linuxtx.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	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>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code
Date: Tue, 9 Jan 2018 22:11:02 -0600	[thread overview]
Message-ID: <CAFxkdAqjuW+z1NgjOcbJEjsKXsNnqNd6xoWc4aJYk9_dtcmwtA@mail.gmail.com> (raw)
In-Reply-To: <ce6c99be-b8ea-f789-ca7c-625c99b2a7a2@intel.com>

On Tue, Jan 9, 2018 at 8:02 PM, Dave Hansen <dave.hansen@intel.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.