Re: [PATCH v3 02/13] x86/xen: simplify flush_lazy_mmu()

Kevin Brodsky
2025-10-16T07:32:29Z

On 15/10/2025 18:52, Dave Hansen wrote:
> On 10/15/25 01:27, Kevin Brodsky wrote:
>> While at it, we can also avoid preempt_disable() if we are not
>> in lazy MMU mode - xen_get_lazy_mode() should tolerate preemption.
> ...
>> 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