On 8/3/21 2:32 PM, Bae, Chang Seok wrote:

quoted

quoted

+static inline void idle_tile(void)
+{
+    if (boot_cpu_has(X86_FEATURE_XGETBV1) && (xgetbv(1) & XFEATURE_MASK_XTILE)) {
+            tile_release();
+            fpregs_deactivate(&current->thread.fpu);
+    }
+}

This isn't obviously safe code.  There's a window in there when we have
bogus, destroyed FPU register state but where we might be rescheduled.

I would assume that preempt is off *somewhere* in this, but it would be
nice to make sure of that, or at least mention the requirement for it to
be off before this code is safe.

I can see preempt_disable() in this path:

$kernel/sched/idle.c::play_idle_precise()
--> preempt_disable()
...
--> do_idle()
    --> cpuidle_idle_call()
        --> call_cpuidle()
            --> $drivers/cpuidle/cpuidle.c::cpuidle_enter()
                --> cpuidle_enter_state()
                    --> target_state->enter()
                        --> $drivers/idle/intel_idle.c::intel_idle_tile()
                            --> idle_tile()
...
--> preempt_enable()

OK, that's good.  Can we comment about the preempt requirement
somewhere?  Or, maybe add a !in_atomic() warning?

Also, should this have something like a fpregs_state_valid() check?  If
the registers are invalid, should this be calling tile_release()?