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
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 708c87b..0917c7f 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);
