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

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Date:   Thu, 4 Oct 2018 10:02:48 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [RFC] x86/cpu_entry_area: move part of it back to fixmap

On Thu, Oct 4, 2018 at 9:31 AM Nadav Amit <namit@...are.com> wrote:
>
> 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.
>

I think you should at least try to benchmark your code against mine,
since you more or less implemented the alternative I suggested. :)