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

[Greg Kurz]

On Wed,  9 Oct 2019 17:08:18 +1100
David Gibson <address@hidden> 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 ? Is it a matter of performance or is it
actually broken or something else ?

What is the impact on -machine accel=kvm,kernel-irqchip=off which
always spit this warning ?

> 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 <address@hidden>
> Cc: Alexey Kardashevskiy <address@hidden>
> 
> Signed-off-by: David Gibson <address@hidden>
> ---
>  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);