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

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Nadav Amit <namit@xxxxxxxxxx>
---

 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);
 }
 
 /*
_