Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

[Julien Grall]

On 08/02/2021 12:14, Jürgen Groß wrote:
> On 08.02.21 11:40, Julien Grall wrote:
> > Hi Juergen,
> >
> > On 08/02/2021 10:22, Jürgen Groß wrote:
> > > On 08.02.21 10:54, Julien Grall wrote:
> > > > ... I don't really see how the difference matter here. The idea is 
> > > > to re-use what's already existing rather than trying to re-invent 
> > > > the wheel with an extra lock (or whatever we can come up).
> > >
> > > The difference is that the race is occurring _before_ any IRQ is
> > > involved. So I don't see how modification of IRQ handling would help.
> >
> > Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:
> >
> > if ( irq in progress )
> > {
> >    set IRQS_PENDING
> >    return;
> > }
> >
> > do
> > {
> >    clear IRQS_PENDING
> >    handle_irq()
> > } while (IRQS_PENDING is set)
> >
> > IRQ handling flow like handle_fasteoi_irq() looks like:
> >
> > if ( irq in progress )
> >    return;
> >
> > handle_irq()
> >
> > The latter flow would catch "spurious" interrupt and ignore them. So 
> > it would handle nicely the race when changing the event affinity.
>
> Sure? Isn't "irq in progress" being reset way before our "lateeoi" is
> issued, thus having the same problem again? 

Sorry I can't parse this.

And I think we want to keep
> the lateeoi behavior in order to be able to control event storms.

I didn't (yet) suggest to remove lateeoi. I only suggest to use a 
different workflow to handle the race with vCPU affinity.

Cheers,

--
Julien Grall