linux-kernel - Re: [PATCH 0/3] Use global pages with PTI

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Message-ID: <CA+55aFy8k_zSJ_ASyzkA9C-jLV4mZsHpv1sOxJ9qpvfS_P6eMg@mail.gmail.com>
Date:   Thu, 15 Feb 2018 09:47:10 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jürgen Groß <jgross@...e.com>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH 0/3] Use global pages with PTI

On Thu, Feb 15, 2018 at 5:20 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
>
> During the switch over to PTI, we seem to have lost our ability to have
> GLOBAL mappings.

Oops. Odd, I have this distinct memory of somebody even _testing_ the
global bit performance when I pointed out that we shouldn't just make
the bit go away entirely.

[ goes back and looks at archives ]

Oh, that was in fact you who did that performance test.

Heh. Anyway, back then you claimed a noticeable improvement on that
will-it-scale test (although a bigger one when pcid wasn't available),
so yes, if we lost the "global pages for the shared user/kernel
mapping" bit we should definitely get this fixed.

Did you perhaps re-run any benchmark numbers just to verify? Because
it's always good to back up patches that should improve performance
with actual numbers..

           Linus