From: Dave Hansen <dave.hansen@linux.intel.com>

First, it's nice to remove the magic numbers.

Second, KERNEL_PAGE_TABLE_ISOLATION is going to consume half of the
available ASID space.  The space is currently unused, but add a comment to
spell out this new restriction.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: keescook@google.com
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: moritz.lipp@iaik.tugraz.at
Cc: linux-mm@kvack.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: hughd@google.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: michael.schwarz@iaik.tugraz.at
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: richard.fellner@student.tugraz.at
Link: https://lkml.kernel.org/r/20171123003504.57EDB845@viggo.jf.intel.com

---
 arch/x86/include/asm/tlbflush.h |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -75,6 +75,22 @@ static inline u64 inc_mm_tlb_gen(struct
 	return new_tlb_gen;
 }
 
+/* There are 12 bits of space for ASIDS in CR3 */
+#define CR3_HW_ASID_BITS		12
+/*
+ * When enabled, KERNEL_PAGE_TABLE_ISOLATION consumes a single bit for
+ * user/kernel switches
+ */
+#define KPTI_CONSUMED_ASID_BITS		0
+
+#define CR3_AVAIL_ASID_BITS (CR3_HW_ASID_BITS - KPTI_CONSUMED_ASID_BITS)
+/*
+ * ASIDs are zero-based: 0->MAX_AVAIL_ASID are valid.  -1 below to account
+ * for them being zero-based.  Another -1 is because ASID 0 is reserved for
+ * use by non-PCID-aware users.
+ */
+#define MAX_ASID_AVAILABLE ((1 << CR3_AVAIL_ASID_BITS) - 2)
+
 /*
  * If PCID is on, ASID-aware code paths put the ASID+1 into the PCID bits.
  * This serves two purposes.  It prevents a nasty situation in which
@@ -87,7 +103,7 @@ struct pgd_t;
 static inline unsigned long build_cr3(pgd_t *pgd, u16 asid)
 {
 	if (static_cpu_has(X86_FEATURE_PCID)) {
-		VM_WARN_ON_ONCE(asid > 4094);
+		VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
 		return __sme_pa(pgd) | (asid + 1);
 	} else {
 		VM_WARN_ON_ONCE(asid != 0);
@@ -97,7 +113,7 @@ static inline unsigned long build_cr3(pg
 
 static inline unsigned long build_cr3_noflush(pgd_t *pgd, u16 asid)
 {
-	VM_WARN_ON_ONCE(asid > 4094);
+	VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
 	return __sme_pa(pgd) | (asid + 1) | CR3_NOFLUSH;
 }