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 <[email protected]>
Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

2 files changed, 6 insertions, 8 deletions

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 708c87b88cc1..0917c7f25720 100644
--- a/arch/x86/kernel/process_32.c
+++ b/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);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 33b268747bb7..1553e19904e0 100644
--- a/arch/x86/kernel/process_64.c
+++ b/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);