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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 2 Apr 2018 10:56:26 -0700
Message-ID: <CA+55aFx5GCahkr_-Y0qF5S=USCXhNcvWZ6gr_TxpvUVAh46STA@mail.gmail.com>
Subject: Re: [PATCH 09/11] x86/pti: enable global pages for shared areas
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Sender: owner-linux-mm@kvack.org
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To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andrea Arcangeli <aarcange@redhat.com>, Andrew Lutomirski <luto@kernel.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>, the arch/x86 maintainers <x86@kernel.org>, namit@vmware.com

On Mon, Apr 2, 2018 at 10:27 AM, Dave Hansen
<dave.hansen@linux.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