[RFC][PATCH 09/10] x86/pti: enable global pages for shared areas

Dave Hansen
2018-02-22T20:37:21Z

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 | 9 ++++++++-
2 files changed, 17 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 2018-02-22 12:36:22.067036545 -0800
+++ b/arch/x86/mm/cpu_entry_area.c 2018-02-22 12:36:22.072036545 -0800
@@ -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);
+
+ 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 2018-02-22 12:36:22.069036545 -0800
+++ b/arch/x86/mm/pti.c 2018-02-22 12:36:22.072036545 -0800
@@ -300,6 +300,13 @@ 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.
+ */
+ *pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);
+
+ /*
* Copy the PMD. That is, the kernelmode and usermode
* tables will share the last-level page tables of this
* address range
@@ -348,7 +355,7 @@ static void __init pti_clone_entry_text(
{
pti_clone_pmds((unsigned long) __entry_text_start,
(unsigned long) __irqentry_text_end,
- _PAGE_RW | _PAGE_GLOBAL);
+ _PAGE_RW);
}

/*
_