Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable()

[Jan Beulich]

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