linux-kernel - Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

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Date:   Mon, 2 Apr 2018 10:56:26 -0700
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>,
        Andrea Arcangeli <aarcange@...hat.com>,
        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>, namit@...are.com
Subject: Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

On Mon, Apr 2, 2018 at 10:27 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
>
> +       /*
> +        * The cpu_entry_area is shared between the user and kernel
> +        * page tables.  All of its ptes can safely be global.
> +        */
> +       if (boot_cpu_has(X86_FEATURE_PGE))
> +               pte = pte_set_flags(pte, _PAGE_GLOBAL);

So this is where the quesion of "why is this conditional" is valid.

We could set _PAGE_GLOBAL unconditionally, not bothering with testing
X86_FEATURE_PGE.

                Linus