x86/fpu: Remove task_disable_lazy_fpu_restore()

Replace task_disable_lazy_fpu_restore() with easier to read
open-coded uses: we already update the fpu->last_cpu field
explicitly in other cases.

(This also removes yet another task_struct using FPU method.)

Better explain the fpu::last_cpu field in the structure definition.
Reviewed-by: NBorislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/include/asm/fpu-internal.h

@@ -74,16 +74,6 @@ static inline void __cpu_disable_lazy_restore(unsigned int cpu)
 	per_cpu(fpu_fpregs_owner_ctx, cpu) = NULL;
 }
 
-/*
- * Used to indicate that the FPU state in memory is newer than the FPU
- * state in registers, and the FPU state should be reloaded next time the
- * task is run. Only safe on the current task, or non-running tasks.
- */
-static inline void task_disable_lazy_fpu_restore(struct task_struct *tsk)
-{
-	tsk->thread.fpu.last_cpu = ~0;
-}
-
 static inline int fpu_lazy_restore(struct task_struct *new, unsigned int cpu)
 {
 	return &new->thread.fpu == this_cpu_read_stable(fpu_fpregs_owner_ctx) &&
@@ -430,7 +420,7 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 
 	if (old_fpu->has_fpu) {
 		if (!fpu_save_init(&old->thread.fpu))
-			task_disable_lazy_fpu_restore(old);
+			old->thread.fpu.last_cpu = -1;
 		else
 			old->thread.fpu.last_cpu = cpu;
 
@@ -446,7 +436,7 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 			stts();
 	} else {
 		old->thread.fpu.counter = 0;
-		task_disable_lazy_fpu_restore(old);
+		old->thread.fpu.last_cpu = -1;
 		if (fpu.preload) {
 			new->thread.fpu.counter++;
 			if (fpu_lazy_restore(new, cpu))

arch/x86/include/asm/fpu/types.h

@@ -125,7 +125,18 @@ union thread_xstate {
 };
 
 struct fpu {
+	/*
+	 * Records the last CPU on which this context was loaded into
+	 * FPU registers. (In the lazy-switching case we might be
+	 * able to reuse FPU registers across multiple context switches
+	 * this way, if no intermediate task used the FPU.)
+	 *
+	 * A value of -1 is used to indicate that the FPU state in context
+	 * memory is newer than the FPU state in registers, and that the
+	 * FPU state should be reloaded next time the task is run.
+	 */
 	unsigned int			last_cpu;
+
 	unsigned int			has_fpu;
 	union thread_xstate		*state;
 	/*

arch/x86/kernel/fpu/core.c

@@ -242,8 +242,7 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)
 	dst->thread.fpu.counter = 0;
 	dst->thread.fpu.has_fpu = 0;
 	dst->thread.fpu.state = NULL;
-
-	task_disable_lazy_fpu_restore(dst);
+	dst->thread.fpu.last_cpu = -1;
 
 	if (src_fpu->fpstate_active) {
 		int err = fpstate_alloc(dst_fpu);
@@ -319,7 +318,7 @@ static int fpu__unlazy_stopped(struct task_struct *child)
 		return -EINVAL;
 
 	if (child_fpu->fpstate_active) {
-		task_disable_lazy_fpu_restore(child);
+		child->thread.fpu.last_cpu = -1;
 		return 0;
 	}