x86/mm: Use/Fix PCID to optimize user/kernel switches

We can use PCID to retain the TLBs across CR3 switches; including those now
part of the user/kernel switch. This increases performance of kernel
entry/exit at the cost of more expensive/complicated TLB flushing.

Now that we have two address spaces, one for kernel and one for user space,
we need two PCIDs per mm. We use the top PCID bit to indicate a user PCID
(just like we use the PFN LSB for the PGD). Since we do TLB invalidation
from kernel space, the existing code will only invalidate the kernel PCID,
we augment that by marking the corresponding user PCID invalid, and upon
switching back to userspace, use a flushing CR3 write for the switch.

In order to access the user_pcid_flush_mask we use PER_CPU storage, which
means the previously established SWAPGS vs CR3 ordering is now mandatory
and required.

Having to do this memory access does require additional registers, most
sites have a functioning stack and we can spill one (RAX), sites without
functional stack need to otherwise provide the second scratch register.

Note: PCID is generally available on Intel Sandybridge and later CPUs.
Note: Up until this point TLB flushing was broken in this series.

Based-on-code-from: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 79 insertions, 12 deletions

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 490a706..5dcc38b 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -10,6 +10,8 @@
 #include <asm/special_insns.h>
 #include <asm/smp.h>
 #include <asm/invpcid.h>
+#include <asm/pti.h>
+#include <asm/processor-flags.h>
 
 static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
 {
@@ -24,24 +26,54 @@ static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
 
 /* There are 12 bits of space for ASIDS in CR3 */
 #define CR3_HW_ASID_BITS		12
+
 /*
  * When enabled, PAGE_TABLE_ISOLATION consumes a single bit for
  * user/kernel switches
  */
-#define PTI_CONSUMED_ASID_BITS		0
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+# define PTI_CONSUMED_PCID_BITS	1
+#else
+# define PTI_CONSUMED_PCID_BITS	0
+#endif
+
+#define CR3_AVAIL_PCID_BITS (X86_CR3_PCID_BITS - PTI_CONSUMED_PCID_BITS)
 
-#define CR3_AVAIL_ASID_BITS (CR3_HW_ASID_BITS - PTI_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)
+#define MAX_ASID_AVAILABLE ((1 << CR3_AVAIL_PCID_BITS) - 2)
+
+/*
+ * 6 because 6 should be plenty and struct tlb_state will fit in two cache
+ * lines.
+ */
+#define TLB_NR_DYN_ASIDS	6
 
 static inline u16 kern_pcid(u16 asid)
 {
 	VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
+
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+	/*
+	 * Make sure that the dynamic ASID space does not confict with the
+	 * bit we are using to switch between user and kernel ASIDs.
+	 */
+	BUILD_BUG_ON(TLB_NR_DYN_ASIDS >= (1 << X86_CR3_PTI_SWITCH_BIT));
+
 	/*
+	 * The ASID being passed in here should have respected the
+	 * MAX_ASID_AVAILABLE and thus never have the switch bit set.
+	 */
+	VM_WARN_ON_ONCE(asid & (1 << X86_CR3_PTI_SWITCH_BIT));
+#endif
+	/*
+	 * The dynamically-assigned ASIDs that get passed in are small
+	 * (<TLB_NR_DYN_ASIDS).  They never have the high switch bit set,
+	 * so do not bother to clear it.
+	 *
 	 * 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 PCID-unaware code saves CR3, loads some other
@@ -95,12 +127,6 @@ static inline bool tlb_defer_switch_to_init_mm(void)
 	return !static_cpu_has(X86_FEATURE_PCID);
 }
 
-/*
- * 6 because 6 should be plenty and struct tlb_state will fit in
- * two cache lines.
- */
-#define TLB_NR_DYN_ASIDS 6
-
 struct tlb_context {
 	u64 ctx_id;
 	u64 tlb_gen;
@@ -146,6 +172,13 @@ struct tlb_state {
 	bool invalidate_other;
 
 	/*
+	 * Mask that contains TLB_NR_DYN_ASIDS+1 bits to indicate
+	 * the corresponding user PCID needs a flush next time we
+	 * switch to it; see SWITCH_TO_USER_CR3.
+	 */
+	unsigned short user_pcid_flush_mask;
+
+	/*
 	 * Access to this CR4 shadow and to H/W CR4 is protected by
 	 * disabling interrupts when modifying either one.
 	 */
@@ -250,14 +283,41 @@ static inline void cr4_set_bits_and_update_boot(unsigned long mask)
 extern void initialize_tlbstate_and_flush(void);
 
 /*
+ * Given an ASID, flush the corresponding user ASID.  We can delay this
+ * until the next time we switch to it.
+ *
+ * See SWITCH_TO_USER_CR3.
+ */
+static inline void invalidate_user_asid(u16 asid)
+{
+	/* There is no user ASID if address space separation is off */
+	if (!IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION))
+		return;
+
+	/*
+	 * We only have a single ASID if PCID is off and the CR3
+	 * write will have flushed it.
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_PCID))
+		return;
+
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	__set_bit(kern_pcid(asid),
+		  (unsigned long *)this_cpu_ptr(&cpu_tlbstate.user_pcid_flush_mask));
+}
+
+/*
  * flush the entire current user mapping
  */
 static inline void __native_flush_tlb(void)
 {
+	invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
 	/*
-	 * If current->mm == NULL then we borrow a mm which may change during a
-	 * task switch and therefore we must not be preempted while we write CR3
-	 * back:
+	 * If current->mm == NULL then we borrow a mm which may change
+	 * during a task switch and therefore we must not be preempted
+	 * while we write CR3 back:
 	 */
 	preempt_disable();
 	native_write_cr3(__native_read_cr3());
@@ -301,7 +361,14 @@ static inline void __native_flush_tlb_global(void)
  */
 static inline void __native_flush_tlb_single(unsigned long addr)
 {
+	u32 loaded_mm_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
+
 	asm volatile("invlpg (%0)" ::"r" (addr) : "memory");
+
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	invalidate_user_asid(loaded_mm_asid);
 }
 
 /*