linux-kernel - Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

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Date:   Thu, 25 Jan 2018 13:20:49 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <alan@...ux.intel.com>, Jann Horn <jannh@...gle.com>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

On Thu, Jan 25, 2018 at 1:08 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> With retpoline, the retpoline in the trampoline sucks.  I don't need
> perf for that -- I've benchmarked it both ways.  It sucks.  I'll fix
> it, but it'll be kind of complicated.

Ahh, I'd forgotten about that (and obviously didn't see it in the profiles).

But yeah, that is fixable even if it does require a page per CPU. Or
did you have some clever scheme in mind?

But that's independent of the slow/fast path.

          Linus