On 15/10/2025 18:52, Dave Hansen wrote:

On 10/15/25 01:27, Kevin Brodsky wrote:

quoted

While at it, we can also avoid preempt_disable() if we are not
in lazy MMU mode - xen_get_lazy_mode() should tolerate preemption.

...

quoted

 static void xen_flush_lazy_mmu(void)
 {
-	preempt_disable();
-
 	if (xen_get_lazy_mode() == XEN_LAZY_MMU) {
-		arch_leave_lazy_mmu_mode();
-		arch_enter_lazy_mmu_mode();
+		preempt_disable();
+		xen_mc_flush();
+		preempt_enable();
 	}

But xen_get_lazy_mode() does:

	this_cpu_read(xen_lazy_mode);

Couldn't preemption end up doing the 'xen_lazy_mode' read and the
xen_mc_flush() on different CPUs?

That seems like a problem. Is there a reason it's safe?

You're right, I was thinking in terms of task, but xen_mc_flush() does
operate on the current CPU (and it's called when context-switching).
Will restore the original order in v4.

- Kevin