linux-kernel - Re: [PATCH v12 1/8] x86/cet/ibt: Add Kconfig option for user-mode Indirect Branch Tracking

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Date:   Mon, 21 Sep 2020 15:41:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>, Pavel Machek <pavel@....cz>
Cc:     Randy Dunlap <rdunlap@...radead.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v12 1/8] x86/cet/ibt: Add Kconfig option for user-mode
 Indirect Branch Tracking

On 9/21/20 3:30 PM, Yu, Yu-cheng wrote:
> +config X86_INTEL_BRANCH_TRACKING_USER
> +    prompt "Intel Indirect Branch Tracking for user-mode" 

Take the "Intel " and "INTEL_" out, please.  It will only cause us all
pain later if some of our x86 compatriots decide to implement this.

> If the kernel is to be used only on older systems that do not support
> IBT, and the size of the binary is important, you can save 900 KB by
> disabling this feature.
> 
> Otherwise, if unsure, say Y.

900k seems like a *lot*.  Where the heck does that come from?

Also, comments like that don't age very well.  Consider:

	Support for this feature is only known to be present on Intel
	processors released in 2020 or later.  This feature is also
	known to increase kernel text size substantially.

	If unsure, say N.

The 900KB is probably wrong today in a lot of configurations, and will;
only get *more* wrong over time.