Re: Xen regression, Was: [PATCH] x86/irq: Probe for PIC presence before allocating descs for legacy IRQs

[Boris Ostrovsky]

On 04/12/2016 09:27 PM, Boris Ostrovsky wrote:
> On 04/12/2016 07:15 PM, Stefano Stabellini wrote:
> > On Tue, 12 Apr 2016, Boris Ostrovsky wrote:
> > > On 04/12/2016 05:56 PM, Stefano Stabellini wrote:
> > > >    I am not sure, maybe you didn't have CONFIG_SPARSE_IRQ?
> > > > But I am certain that 4.6-rc2, with the attached config, fails as Dom0
> > > > on QEMU with the following sequence of calls:
> > > I did have CONFIG_SPARSE_IRQ and I just rebuilt 4.5.0 with your config
> > > (4.6-rc3 doesn't build for me for some reason) and that booted dom0 
> > > as well.
> > >
> > > BTW, what do you mean by "dom0 on QEMU"?
> >   I am running Xen and Linux inside a QEMU x86_64 emulated machine 
> > (nested
> > virt).
>
> This I, of course, never tried.
>
> But given that things work in a single-level virt, doesn't this imply 
> that perhaps there is something in the emulation that's not quite right?

OK, so this *is* broken on single level virt as well. It's just that we 
always end up using AHCI so lack of irq 14 (and 15) does not affect the 
system. And I guess in QEMU case it's IDE only, right?

You patch does fix this but I wonder if we could change something in 
probe_8259A() so that we can continue using nr_legacy_irqs(). Using 
nr_legacy_irqs() and NR_IRQS_LEGACY at the same time is inconsistent and 
may cause us headaches in the future.


-boris