Re: [PATCH v4 19/19] spapr: Work around spurious warnings from vfio INTx initialization

[Greg Kurz <groug@kaod.org>]

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On Wed, 9 Oct 2019 19:52:59 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Oct 09, 2019 at 10:37:38AM +0200, Greg Kurz wrote:
> > On Wed,  9 Oct 2019 17:08:18 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> > 
> > > Traditional PCI INTx for vfio devices can only perform well if using
> > > an in-kernel irqchip.  Therefore, vfio_intx_update() issues a warning
> > > if an in kernel irqchip is not available.
> > 
> > Can you elaborate on what doesn't "perform well" without an
> > in-kernel irqchip ?
> 
> Not really, no, but Alex Williamson tells me it is soo.
> 
> > Is it a matter of performance or is it
> > actually broken or something else ?
> 
> I believe it's a matter of performance, but such a big one that it
> makes using it without kernel irqchip infeasible in many cases.
> 
> > What is the impact on -machine accel=kvm,kernel-irqchip=off which
> > always spit this warning ?
> 
> It should still spit that warning.
> 

Yeah of course it does, but it is expected that we cannot setup
the irqfd since we deliberately don't have an in-kernel irqchip,
isn't it ? Why spit a warning in this case ? Or is it just a not
very user friendly way of saying "you will have poor performance
if the VFIO device uses PCI INTx" ?

> > > We usually do have an in-kernel irqchip available for pseries machines
> > > on POWER hosts.  However, because the platform allows feature
> > > negotiation of what interrupt controller model to use, we don't
> > > currently initialize it until machine reset.  vfio_intx_update() is
> > > called (first) from vfio_realize() before that, so it can issue a
> > > spurious warning, even if we will have an in kernel irqchip by the
> > > time we need it.
> > > 
> > > To workaround this, make a call to spapr_irq_update_active_intc() from
> > > spapr_irq_init() which is called at machine realize time, before the
> > > vfio realize.  This call will be pretty much obsoleted by the later
> > > call at reset time, but it serves to suppress the spurious warning
> > > from VFIO.
> > > 
> > > Cc: Alex Williamson <alex.williamson@redhat.com>
> > > Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
> > > 
> > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > ---
> > >  hw/ppc/spapr_irq.c | 11 ++++++++++-
> > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/ppc/spapr_irq.c b/hw/ppc/spapr_irq.c
> > > index 7964e4a1b8..3aeb523f3e 100644
> > > --- a/hw/ppc/spapr_irq.c
> > > +++ b/hw/ppc/spapr_irq.c
> > > @@ -274,6 +274,14 @@ void spapr_irq_init(SpaprMachineState *spapr, Error **errp)
> > >  
> > >      spapr->qirqs = qemu_allocate_irqs(spapr_set_irq, spapr,
> > >                                        smc->nr_xirqs + SPAPR_XIRQ_BASE);
> > > +
> > > +    /*
> > > +     * Mostly we don't actually need this until reset, except that not
> > > +     * having this set up can cause VFIO devices to issue a
> > > +     * false-positive warning during realize(), because they don't yet
> > > +     * have an in-kernel irq chip.
> > > +     */
> > > +    spapr_irq_update_active_intc(spapr);
> > >  }
> > >  
> > >  int spapr_irq_claim(SpaprMachineState *spapr, int irq, bool lsi, Error **errp)
> > > @@ -429,7 +437,8 @@ void spapr_irq_update_active_intc(SpaprMachineState *spapr)
> > >           * this.
> > >           */
> > >          new_intc = SPAPR_INTC(spapr->xive);
> > > -    } else if (spapr_ovec_test(spapr->ov5_cas, OV5_XIVE_EXPLOIT)) {
> > > +    } else if (spapr->ov5_cas
> > > +               && spapr_ovec_test(spapr->ov5_cas, OV5_XIVE_EXPLOIT)) {
> > >          new_intc = SPAPR_INTC(spapr->xive);
> > >      } else {
> > >          new_intc = SPAPR_INTC(spapr->ics);
> > 
> 


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