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

[Boris Ostrovsky]

On 04/14/2016 09:45 AM, David Vrabel wrote:
> On 12/04/16 19:06, Stefano Stabellini wrote:
> > On Tue, 12 Apr 2016, Boris Ostrovsky wrote:
> > > On 04/11/2016 10:08 PM, Stefano Stabellini wrote:
> > > > Hi all,
> > > >
> > > > Unfortunately this patch (now commit
> > > > 8c058b0b9c34d8c8d7912880956543769323e2d8) causes a regression on Xen
> > > > when running on top of QEMU: the number of PIT irqs get set to 0 by
> > > > probe_8259A but actually there are 16.
> > > >
> > > > Any suggestions on how to fix this?
> > > >
> > > > 1) we could revert 8c058b0b9c34d8c8d7912880956543769323e2d8
> > > > 2) we could introduce an 'if (!xen_domain())' in probe_8259A
> > > > 3) suggestions welcome
> > > Stefano, do you have b4ff8389ed14b849354b59ce9b360bdefcdbf99c ?
> > >
> > > It was supposed to fix this problem for Xen. However, I just noticed that
> > > arch/arm64/include/asm/irq.h makes nr_legacy_irqs() return 0 (unlike
> > > arch/arm/include/asm/irq.h). Could that be the problem?
> > I have b4ff8389ed14b849354b59ce9b360bdefcdbf99c but it doesn't fix the
> > issue for me.
> >
> > Is the idea of your patch that xen_allocate_irq_gsi will allocate the
> > descriptor dynamically instead?  If so, it doesn't work because it
> > doesn't get called for irq 14:
> >
> > piix_init_one -> ata_pci_sff_activate_host -> devm_request_irq ->
> > devm_request_threaded_irq-> request_threaded_irq -> irq_to_desc(14) ->
> > -EVAIL
> >
> > If you look at pci_xen_initial_domain, the loop:
> >
> > for (irq = 0; irq < nr_legacy_irqs(); irq++) {
> >
> > won't work anymore because by the time is called, nr_legacy_irqs()
> > already returns 0, because it has been changed by probe_8259A().
> >
> > We also need the following patch:
> >
> > ---
> >
> > xen/x86: actually allocate legacy interrupts on PV guests
> >
> > b4ff8389ed14 is incomplete: relies on nr_legacy_irqs() to get the number
> > of legacy interrupts when actually nr_legacy_irqs() returns 0 after
> > probe_8259A(). Use NR_IRQS_LEGACY instead.
> >
> > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> >
> > diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
> > index beac4df..6db0060 100644
> > --- a/arch/x86/pci/xen.c
> > +++ b/arch/x86/pci/xen.c
> > @@ -492,7 +492,7 @@ int __init pci_xen_initial_domain(void)
> >   	__acpi_register_gsi = acpi_register_gsi_xen;
> >   	__acpi_unregister_gsi = NULL;
> >   	/* Pre-allocate legacy irqs */
> > -	for (irq = 0; irq < nr_legacy_irqs(); irq++) {
> > +	for (irq = 0; irq < NR_IRQS_LEGACY; irq++) {
> >   		int trigger, polarity;
> >   
> >   		if (acpi_get_override_irq(irq, &trigger, &polarity) == -1)
> I think I prefer this fix because PV guests don't have a legacy PIC and
> don't need any legacy irqs allocated so fiddling with nr_legacy_irqs()
> seems wrong.
>
> But we do need a comment here saying something like:
>
> /*
>   * Pre-allocate the legacy IRQs.  Use NR_LEGACY_IRQS here
>   * because we don't have a PCI and thus nr_legacy_irqs() is zero.

s/PCI/PIC


>   */
>
> Does this make sense?
>
> David

OK. (I first thought that we also don't have RTC and yet we claim that 
dom0 has it. But then I realized that we do emulate it)

-boris