linux-kernel - Re: [PATCH v14 1/7] x86/cet/ibt: Add Kconfig option for user-mode Indirect Branch Tracking

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Date:   Mon, 12 Oct 2020 13:29:57 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     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>,
        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>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...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>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v14 1/7] x86/cet/ibt: Add Kconfig option for user-mode
 Indirect Branch Tracking

On 10/12/2020 12:15 PM, Cyrill Gorcunov wrote:
> On Mon, Oct 12, 2020 at 08:45:24AM -0700, Yu-cheng Yu wrote:
> ...
>> +	  the application support it.  When this feature is enabled,
>> +	  legacy non-IBT applications continue to work, but without
>> +	  IBT protection.
>> +	  Support for this feature is only known to be present on
>> +	  processors released in 2020 or later.  CET features are also
>> +	  known to increase kernel text size by 3.7 KB.
> 
> It seems the last sentence is redundant - new features always bloat
> the kernel code and precise size may differ depending on compiler
> and options. Surely this can be patched on top.
> 

This was added after some discussion in v12 about kernel text size [1]. 
I think these few extra words can help people who have older machines 
and want to save some space.  Yes, like you said, if later we don't want 
this sentence, I will be happy to add a patch to remove it.  Thanks for 
the feedback.

Yu-cheng

[1] 
https://lore.kernel.org/linux-api/5e0a4005-45e3-3e88-e6e0-4ec31aad7eb9@intel.com/