x86/fpu: Clean up FPU switching in the middle of task switching

It happens to work, but it's very very wrong, because our 'current'
macro is magic that is supposedly loading a stable value.

It just happens to be not quite stable enough and the compilers
re-load the value enough for this code to work.  But it's wrong.

The whole

        struct fpu *prev_fpu = &prev->fpu;

thing in __switch_to() is pretty ugly. There's no reason why we
should look at that 'prev_fpu' pointer there, or pass it down.

And it only generates worse code, in how it loads 'current' when
__switch_to() has the right task pointers.

The attached patch not only cleans this up, it actually
generates better code too:

 (a) it removes one push/pop pair at entry/exit because there's one
     less register used (no 'current')

 (b) it removes that pointless load of 'current' because it just uses
     the right argument:

	-       movq    %gs:pcpu_hot(%rip), %r12
	-       testq   $16384, (%r12)
	+       testq   $16384, (%rdi)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231018184227.446318-1-ubizjak@gmail.com

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

@@ -37,10 +37,12 @@ extern void fpu_flush_thread(void);
  * The FPU context is only stored/restored for a user task and
  * PF_KTHREAD is used to distinguish between kernel and user threads.
  */
-static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
+static inline void switch_fpu_prepare(struct task_struct *old, int cpu)
 {
 	if (cpu_feature_enabled(X86_FEATURE_FPU) &&
-	    !(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
+	    !(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
+		struct fpu *old_fpu = &old->thread.fpu;
+
 		save_fpregs_to_fpstate(old_fpu);
 		/*
 		 * The save operation preserved register state, so the
@@ -60,10 +62,10 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
  * Delay loading of the complete FPU state until the return to userland.
  * PKRU is handled separately.
  */
-static inline void switch_fpu_finish(void)
+static inline void switch_fpu_finish(struct task_struct *new)
 {
 	if (cpu_feature_enabled(X86_FEATURE_FPU))
-		set_thread_flag(TIF_NEED_FPU_LOAD);
+		set_tsk_thread_flag(new, TIF_NEED_FPU_LOAD);
 }
 
 #endif /* _ASM_X86_FPU_SCHED_H */

arch/x86/kernel/process_32.c

@@ -156,13 +156,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 {
 	struct thread_struct *prev = &prev_p->thread,
 			     *next = &next_p->thread;
-	struct fpu *prev_fpu = &prev->fpu;
 	int cpu = smp_processor_id();
 
 	/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */
 
-	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
-		switch_fpu_prepare(prev_fpu, cpu);
+	if (!test_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD))
+		switch_fpu_prepare(prev_p, cpu);
 
 	/*
 	 * Save away %gs. No need to save %fs, as it was saved on the
@@ -209,7 +208,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 
 	raw_cpu_write(pcpu_hot.current_task, next_p);
 
-	switch_fpu_finish();
+	switch_fpu_finish(next_p);
 
 	/* Load the Intel cache allocation PQR MSR. */
 	resctrl_sched_in(next_p);

arch/x86/kernel/process_64.c

@@ -562,14 +562,13 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 {
 	struct thread_struct *prev = &prev_p->thread;
 	struct thread_struct *next = &next_p->thread;
-	struct fpu *prev_fpu = &prev->fpu;
 	int cpu = smp_processor_id();
 
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ENTRY) &&
 		     this_cpu_read(pcpu_hot.hardirq_stack_inuse));
 
-	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
-		switch_fpu_prepare(prev_fpu, cpu);
+	if (!test_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD))
+		switch_fpu_prepare(prev_p, cpu);
 
 	/* We must save %fs and %gs before load_TLS() because
 	 * %fs and %gs may be cleared by load_TLS().
@@ -623,7 +622,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	raw_cpu_write(pcpu_hot.current_task, next_p);
 	raw_cpu_write(pcpu_hot.top_of_stack, task_top_of_stack(next_p));
 
-	switch_fpu_finish();
+	switch_fpu_finish(next_p);
 
 	/* Reload sp0. */
 	update_task_stack(next_p);