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

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Message-ID: <3a0b0baf-a0f5-fd15-3af1-0059a807100b@intel.com>
Date:   Mon, 21 Sep 2020 15:47:50 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...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 9/21/2020 3:41 PM, Dave Hansen wrote:
> 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.
> 

Thanks!

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

I was talking about the vmlinux file, and probably should have said 
bzImage size, which has 14 KB increase when CET is enabled.

Yu-cheng