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

[Juergen Gross]

On 21.09.21 09:53, Jan Beulich wrote:
> 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?

The question is if a stray call in case of preemption (very unlikely)
is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling.

I'm quite sure removing the preempt_*() calls will be a net benefit.


Juergen

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