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Subject: Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI to enable/disable PTI
From: Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20180109220605.GE13282@1wt.eu>
Date: Tue, 9 Jan 2018 15:53:54 -0800
Cc: Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Kees Cook <keescook@chromium.org>
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To: Willy Tarreau <w@1wt.eu>
Sender: linux-kernel-owner@vger.kernel.org


> On Jan 9, 2018, at 2:06 PM, Willy Tarreau <w@1wt.eu> wrote:
> 
>> On Tue, Jan 09, 2018 at 10:46:02PM +0100, Borislav Petkov wrote:
>>> On Tue, Jan 09, 2018 at 10:32:27PM +0100, Willy Tarreau wrote:
>>> Requiring a reboot just to fix a performance problem you've discovered
>>> the hard way is not the most friendly way to help users I'm afraid.
>> 
>> That's a very strange argument: if you know you'd need max perf, you
>> boot with pti=allow_optout.
>> 
>> Color me confused.
> 
> That's very simple : you first know you need more perf when you see the
> name of your boss on your phone asking what's happening with the site
> suddenly crawling at the worst possible moment, when everyone is there
> to see it dead. Performance is something that's tuned at runtime, always,
> not via random reboots. When you have 10 servers running at 100% CPU,
> the last thing you're thinking about is to remove one of them so that
> the 9 remaining ones are at 110% while you reboot :-/

Here's another idea: make it a module
  To enable it, you do modprobe pti_control allow_privileged_prctl=1.