linux-kernel - Re: [RFC] x86/cpu_entry_area: move part of it back to fixmap

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Message-ID: <CE9C4B9C-3B11-45D0-812A-F074D687084D@vmware.com>
Date:   Thu, 4 Oct 2018 16:30:57 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC] x86/cpu_entry_area: move part of it back to fixmap

at 7:11 AM, Andy Lutomirski <luto@...capital.net> wrote:

> 
> 
> On Oct 3, 2018, at 9:59 PM, Nadav Amit <namit@...are.com> wrote:
> 
>> This RFC proposes to return part of the entry-area back to the fixmap to
>> improve system-call performance. Currently, since the entry-area is
>> mapped far (more than 2GB) away from the kernel text, an indirect branch
>> is needed to jump from the trampoline into the kernel. Due to Spectre
>> v2, vulnerable CPUs need to use a retpoline, which introduces an
>> overhead of >20 cycles.
> 
> That retpoline is gone in -tip. Can you see how your code stacks up against -tip?  If it’s enough of a win to justify the added complexity, we can try it. 
> 
> You can see some pros and cons in the changelog:
> 
> https://git.kernel.org/tip/bf904d2762ee6fc1e4acfcb0772bbfb4a27ad8a6

Err.. That’s what I get for not following lkml. Very nice discussion.
Based on it, I may be able to do an additional micro-optimizations or
two. Let me give it a try.