linux-kernel - Re: [PATCH 08/16] x86/pgtable/32: Allocate 8k page-tables when PTI is enabled

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Date:   Wed, 17 Jan 2018 15:43:14 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 08/16] x86/pgtable/32: Allocate 8k page-tables when PTI is enabled

On Tue, Jan 16, 2018 at 8:36 AM, Joerg Roedel <joro@...tes.org> wrote:
> From: Joerg Roedel <jroedel@...e.de>
>
> Allocate a kernel and a user page-table root when PTI is
> enabled. Also allocate a full page per root for PAEm because
> otherwise the bit to flip in cr3 to switch between them
> would be non-constant, which creates a lot of hassle.
> Keep that for a later optimization.
>
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/kernel/head_32.S | 23 ++++++++++++++++++-----
>  arch/x86/mm/pgtable.c     | 11 ++++++-----
>  2 files changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index c29020907886..fc550559bf58 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -512,28 +512,41 @@ ENTRY(initial_code)
>  ENTRY(setup_once_ref)
>         .long setup_once
>
> +#ifdef CONFIG_PAGE_TABLE_ISOLATION
> +#define        PGD_ALIGN       (2 * PAGE_SIZE)
> +#define PTI_USER_PGD_FILL      1024
> +#else
> +#define        PGD_ALIGN       (PAGE_SIZE)
> +#define PTI_USER_PGD_FILL      0
> +#endif
>  /*
>   * BSS section
>   */
>  __PAGE_ALIGNED_BSS
> -       .align PAGE_SIZE
> +       .align PGD_ALIGN
>  #ifdef CONFIG_X86_PAE
>  .globl initial_pg_pmd
>  initial_pg_pmd:
>         .fill 1024*KPMDS,4,0
> +       .fill PTI_USER_PGD_FILL,4,0

Couldn't this be simplified to just .align PGD_ALIGN, 0 without the .fill?