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SubjectRe: [PATCH 09/11] x86/pti: enable global pages for shared areas
Date
Dave Hansen <dave.hansen@linux.intel.com> wrote:

>
> From: Dave Hansen <dave.hansen@linux.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@linux.intel.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Kees Cook <keescook@google.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: x86@kernel.org
> Cc: Nadav Amit <namit@vmware.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.

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