On 21.09.2021 09:02, Juergen Gross wrote:
> --- a/arch/x86/xen/irq.c
> +++ b/arch/x86/xen/irq.c
> @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void)
> {
> struct vcpu_info *vcpu;
>
> - /*
> - * We may be preempted as soon as vcpu->evtchn_upcall_mask is
> - * cleared, so disable preemption to ensure we check for
> - * events on the VCPU we are still running on.
> - */
> - preempt_disable();
> -
> vcpu = this_cpu_read(xen_vcpu);
> vcpu->evtchn_upcall_mask = 0;
>
> - /* Doesn't matter if we get preempted here, because any
> - pending event will get dealt with anyway. */
> + /*
> + * Now preemption could happen, but this is only possible if an event
> + * was handled, so missing an event due to preemption is not
> + * possible at all.
> + * The worst possible case is to be preempted and then check events
> + * pending on the old vcpu, but this is not problematic.
> + */
I agree this isn't problematic from a functional perspective, but ...
> barrier(); /* unmask then check (avoid races) */
> if (unlikely(vcpu->evtchn_upcall_pending))
> xen_force_evtchn_callback();
... is a stray call here cheaper than ...
> -
> - preempt_enable();
... the preempt_{dis,en}able() pair?
Jan
