6 files changed, 143 insertions, 18 deletions

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index f4e524b67e91..7a900142dbc8 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -37,6 +37,8 @@ struct fpsimd_state {
                         u32 fpcr;
                 };
         };
+        /* the id of the last cpu to have restored this state */
+        unsigned int cpu;
 };
 
 #if defined(__KERNEL__) && defined(CONFIG_COMPAT)
@@ -59,8 +61,11 @@ extern void fpsimd_thread_switch(struct task_struct *next);
 extern void fpsimd_flush_thread(void);
 
 extern void fpsimd_preserve_current_state(void);
+extern void fpsimd_restore_current_state(void);
 extern void fpsimd_update_current_state(struct fpsimd_state *state);
 
+extern void fpsimd_flush_task_state(struct task_struct *target);
+
 #endif
 
 #endif
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 720e70b66ffd..4a1ca1cfb2f8 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -100,6 +100,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_SIGPENDING          0
 #define TIF_NEED_RESCHED        1
 #define TIF_NOTIFY_RESUME       2       /* callback before returning to user */
+#define TIF_FOREIGN_FPSTATE     3       /* CPU's FP state is not current's */
 #define TIF_SYSCALL_TRACE       8
 #define TIF_POLLING_NRFLAG      16
 #define TIF_MEMDIE              18      /* is terminating due to OOM killer */
@@ -112,10 +113,11 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_SIGPENDING         (1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED       (1 << TIF_NEED_RESCHED)
 #define _TIF_NOTIFY_RESUME      (1 << TIF_NOTIFY_RESUME)
+#define _TIF_FOREIGN_FPSTATE    (1 << TIF_FOREIGN_FPSTATE)
 #define _TIF_32BIT              (1 << TIF_32BIT)
 
 #define _TIF_WORK_MASK          (_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
-                                 _TIF_NOTIFY_RESUME)
+                                 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE)
 
 #endif /* __KERNEL__ */
 #endif /* __ASM_THREAD_INFO_H */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 39ac630d83de..80464e2fb1a5 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -576,7 +576,7 @@ fast_work_pending:
         str     x0, [sp, #S_X0]                 // returned x0
 work_pending:
         tbnz    x1, #TIF_NEED_RESCHED, work_resched
-        /* TIF_SIGPENDING or TIF_NOTIFY_RESUME case */
+        /* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
         ldr     x2, [sp, #S_PSTATE]
         mov     x0, sp                          // 'regs'
         tst     x2, #PSR_MODE_MASK              // user mode regs?
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 8a97163debc7..5ae89303c3ab 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -35,6 +35,60 @@
 #define FPEXC_IDF       (1 << 7)
 
 /*
+ * In order to reduce the number of times the FPSIMD state is needlessly saved
+ * and restored, we need to keep track of two things:
+ * (a) for each task, we need to remember which CPU was the last one to have
+ *     the task's FPSIMD state loaded into its FPSIMD registers;
+ * (b) for each CPU, we need to remember which task's userland FPSIMD state has
+ *     been loaded into its FPSIMD registers most recently, or whether it has
+ *     been used to perform kernel mode NEON in the meantime.
+ *
+ * For (a), we add a 'cpu' field to struct fpsimd_state, which gets updated to
+ * the id of the current CPU everytime the state is loaded onto a CPU. For (b),
+ * we add the per-cpu variable 'fpsimd_last_state' (below), which contains the
+ * address of the userland FPSIMD state of the task that was loaded onto the CPU
+ * the most recently, or NULL if kernel mode NEON has been performed after that.
+ *
+ * With this in place, we no longer have to restore the next FPSIMD state right
+ * when switching between tasks. Instead, we can defer this check to userland
+ * resume, at which time we verify whether the CPU's fpsimd_last_state and the
+ * task's fpsimd_state.cpu are still mutually in sync. If this is the case, we
+ * can omit the FPSIMD restore.
+ *
+ * As an optimization, we use the thread_info flag TIF_FOREIGN_FPSTATE to
+ * indicate whether or not the userland FPSIMD state of the current task is
+ * present in the registers. The flag is set unless the FPSIMD registers of this
+ * CPU currently contain the most recent userland FPSIMD state of the current
+ * task.
+ *
+ * For a certain task, the sequence may look something like this:
+ * - the task gets scheduled in; if both the task's fpsimd_state.cpu field
+ *   contains the id of the current CPU, and the CPU's fpsimd_last_state per-cpu
+ *   variable points to the task's fpsimd_state, the TIF_FOREIGN_FPSTATE flag is
+ *   cleared, otherwise it is set;
+ *
+ * - the task returns to userland; if TIF_FOREIGN_FPSTATE is set, the task's
+ *   userland FPSIMD state is copied from memory to the registers, the task's
+ *   fpsimd_state.cpu field is set to the id of the current CPU, the current
+ *   CPU's fpsimd_last_state pointer is set to this task's fpsimd_state and the
+ *   TIF_FOREIGN_FPSTATE flag is cleared;
+ *
+ * - the task executes an ordinary syscall; upon return to userland, the
+ *   TIF_FOREIGN_FPSTATE flag will still be cleared, so no FPSIMD state is
+ *   restored;
+ *
+ * - the task executes a syscall which executes some NEON instructions; this is
+ *   preceded by a call to kernel_neon_begin(), which copies the task's FPSIMD
+ *   register contents to memory, clears the fpsimd_last_state per-cpu variable
+ *   and sets the TIF_FOREIGN_FPSTATE flag;
+ *
+ * - the task gets preempted after kernel_neon_end() is called; as we have not
+ *   returned from the 2nd syscall yet, TIF_FOREIGN_FPSTATE is still set so
+ *   whatever is in the FPSIMD registers is not saved to memory, but discarded.
+ */
+static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);
+
+/*
  * Trapped FP/ASIMD access.
  */
 void do_fpsimd_acc(unsigned int esr, struct pt_regs *regs)
@@ -72,41 +126,96 @@ void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
 
 void fpsimd_thread_switch(struct task_struct *next)
 {
-        /* check if not kernel threads */
-        if (current->mm)
+        /*
+         * Save the current FPSIMD state to memory, but only if whatever is in
+         * the registers is in fact the most recent userland FPSIMD state of
+         * 'current'.
+         */
+        if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
                 fpsimd_save_state(&current->thread.fpsimd_state);
-        if (next->mm)
-                fpsimd_load_state(&next->thread.fpsimd_state);
+
+        if (next->mm) {
+                /*
+                 * If we are switching to a task whose most recent userland
+                 * FPSIMD state is already in the registers of *this* cpu,
+                 * we can skip loading the state from memory. Otherwise, set
+                 * the TIF_FOREIGN_FPSTATE flag so the state will be loaded
+                 * upon the next return to userland.
+                 */
+                struct fpsimd_state *st = &next->thread.fpsimd_state;
+
+                if (__this_cpu_read(fpsimd_last_state) == st
+                    && st->cpu == smp_processor_id())
+                        clear_ti_thread_flag(task_thread_info(next),
+                                             TIF_FOREIGN_FPSTATE);
+                else
+                        set_ti_thread_flag(task_thread_info(next),
+                                           TIF_FOREIGN_FPSTATE);
+        }
 }
 
 void fpsimd_flush_thread(void)
 {
-        preempt_disable();
         memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
-        fpsimd_load_state(&current->thread.fpsimd_state);
-        preempt_enable();
+        set_thread_flag(TIF_FOREIGN_FPSTATE);
 }
 
 /*
- * Save the userland FPSIMD state of 'current' to memory
+ * Save the userland FPSIMD state of 'current' to memory, but only if the state
+ * currently held in the registers does in fact belong to 'current'
  */
 void fpsimd_preserve_current_state(void)
 {
         preempt_disable();
-        fpsimd_save_state(&current->thread.fpsimd_state);
+        if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
+                fpsimd_save_state(&current->thread.fpsimd_state);
         preempt_enable();
 }
 
 /*
- * Load an updated userland FPSIMD state for 'current' from memory
+ * Load the userland FPSIMD state of 'current' from memory, but only if the
+ * FPSIMD state already held in the registers is /not/ the most recent FPSIMD
+ * state of 'current'
+ */
+void fpsimd_restore_current_state(void)
+{
+        preempt_disable();
+        if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
+                struct fpsimd_state *st = &current->thread.fpsimd_state;
+
+                fpsimd_load_state(st);
+                this_cpu_write(fpsimd_last_state, st);
+                st->cpu = smp_processor_id();
+        }
+        preempt_enable();
+}
+
+/*
+ * Load an updated userland FPSIMD state for 'current' from memory and set the
+ * flag that indicates that the FPSIMD register contents are the most recent
+ * FPSIMD state of 'current'
  */
 void fpsimd_update_current_state(struct fpsimd_state *state)
 {
         preempt_disable();
         fpsimd_load_state(state);
+        if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
+                struct fpsimd_state *st = &current->thread.fpsimd_state;
+
+                this_cpu_write(fpsimd_last_state, st);
+                st->cpu = smp_processor_id();
+        }
         preempt_enable();
 }
 
+/*
+ * Invalidate live CPU copies of task t's FPSIMD state
+ */
+void fpsimd_flush_task_state(struct task_struct *t)
+{
+        t->thread.fpsimd_state.cpu = NR_CPUS;
+}
+
 #ifdef CONFIG_KERNEL_MODE_NEON
 
 /*
@@ -118,16 +227,19 @@ void kernel_neon_begin(void)
         BUG_ON(in_interrupt());
         preempt_disable();
 
-        if (current->mm)
+        /*
+         * Save the userland FPSIMD state if we have one and if we haven't done
+         * so already. Clear fpsimd_last_state to indicate that there is no
+         * longer userland FPSIMD state in the registers.
+         */
+        if (current->mm && !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE))
                 fpsimd_save_state(&current->thread.fpsimd_state);
+        this_cpu_write(fpsimd_last_state, NULL);
 }
 EXPORT_SYMBOL(kernel_neon_begin);
 
 void kernel_neon_end(void)
 {
-        if (current->mm)
-                fpsimd_load_state(&current->thread.fpsimd_state);
-
         preempt_enable();
 }
 EXPORT_SYMBOL(kernel_neon_end);
@@ -140,12 +252,12 @@ static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
 {
         switch (cmd) {
         case CPU_PM_ENTER:
-                if (current->mm)
+                if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
                         fpsimd_save_state(&current->thread.fpsimd_state);
                 break;
         case CPU_PM_EXIT:
                 if (current->mm)
-                        fpsimd_load_state(&current->thread.fpsimd_state);
+                        set_thread_flag(TIF_FOREIGN_FPSTATE);
                 break;
         case CPU_PM_ENTER_FAILED:
         default:
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 6a8928bba03c..f8700eca24e7 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -517,6 +517,7 @@ static int fpr_set(struct task_struct *target, const struct user_regset *regset,
                 return ret;
 
         target->thread.fpsimd_state.user_fpsimd = newstate;
+        fpsimd_flush_task_state(target);
         return ret;
 }
 
@@ -764,6 +765,7 @@ static int compat_vfp_set(struct task_struct *target,
                 uregs->fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
         }
 
+        fpsimd_flush_task_state(target);
         return ret;
 }
 
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 06448a77ff53..882f01774365 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -413,4 +413,8 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
                 clear_thread_flag(TIF_NOTIFY_RESUME);
                 tracehook_notify_resume(regs);
         }
+
+        if (thread_flags & _TIF_FOREIGN_FPSTATE)
+                fpsimd_restore_current_state();
+
 }