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

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Date:   Wed, 4 Apr 2018 04:45:00 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "keescook@...gle.com" <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The entry/exit text and cpu_entry_area are mapped into userspace and
> the kernel.  But, they are not _PAGE_GLOBAL.  This creates unnecessary
> TLB misses.
> 
> Add the _PAGE_GLOBAL flag for these areas.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Kees Cook <keescook@...gle.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: x86@...nel.org
> Cc: Nadav Amit <namit@...are.com>
> ---
> 
> b/arch/x86/mm/cpu_entry_area.c |   10 +++++++++-
> b/arch/x86/mm/pti.c            |   14 +++++++++++++-
> 2 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff -puN arch/x86/mm/cpu_entry_area.c~kpti-why-no-global arch/x86/mm/cpu_entry_area.c
> --- a/arch/x86/mm/cpu_entry_area.c~kpti-why-no-global	2018-04-02 16:41:17.157605167 -0700
> +++ b/arch/x86/mm/cpu_entry_area.c	2018-04-02 16:41:17.162605167 -0700
> @@ -27,8 +27,16 @@ EXPORT_SYMBOL(get_cpu_entry_area);
> void cea_set_pte(void *cea_vaddr, phys_addr_t pa, pgprot_t flags)
> {
> 	unsigned long va = (unsigned long) cea_vaddr;
> +	pte_t pte = pfn_pte(pa >> PAGE_SHIFT, flags);
> 
> -	set_pte_vaddr(va, pfn_pte(pa >> PAGE_SHIFT, flags));
> +	/*
> +	 * 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);

I think it would be safer to check that the PTE is indeed present before
setting _PAGE_GLOBAL. For example, percpu_setup_debug_store() sets PAGE_NONE
for non-present entries. In this case, since PAGE_NONE and PAGE_GLOBAL use
the same bit, everything would be fine, but it might cause bugs one day.

> +
> +	set_pte_vaddr(va, pte);
> }
> 
> static void __init
> diff -puN arch/x86/mm/pti.c~kpti-why-no-global arch/x86/mm/pti.c
> --- a/arch/x86/mm/pti.c~kpti-why-no-global	2018-04-02 16:41:17.159605167 -0700
> +++ b/arch/x86/mm/pti.c	2018-04-02 16:41:17.163605167 -0700
> @@ -300,6 +300,18 @@ pti_clone_pmds(unsigned long start, unsi
> 			return;
> 
> 		/*
> +		 * Setting 'target_pmd' below creates a mapping in both
> +		 * the user and kernel page tables.  It is effectively
> +		 * global, so set it as global in both copies.  Note:
> +		 * the X86_FEATURE_PGE check is not _required_ because
> +		 * the CPU ignores _PAGE_GLOBAL when PGE is not
> +		 * supported.  The check keeps consistentency with
> +		 * code that only set this bit when supported.
> +		 */
> +		if (boot_cpu_has(X86_FEATURE_PGE))
> +			*pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);

Same here.