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

[Juergen Gross <jgross@xxxxxxxx>]

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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