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

[David Gibson]

On Wed, Oct 09, 2019 at 07:16:39PM +0200, Greg Kurz wrote:
> On Wed, 9 Oct 2019 19:52:59 +1100
> David Gibson <address@hidden> 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 <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 ?
> > 
> > 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" ?

The last thing, AIUI.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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