Re: [Xen-devel] [PATCH] xen/x86: actually allocate legacy interrupts on PV guests

[Boris Ostrovsky]

On 04/27/2016 05:35 AM, David Vrabel wrote:
> On 27/04/16 06:02, Juergen Gross wrote:
> > On 21/04/16 11:30, Stefano Stabellini wrote:
> > > On Thu, 21 Apr 2016, Juergen Gross wrote:
> > > > On 20/04/16 15:15, Stefano Stabellini wrote:
> > > > > 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.
> > > > Would you mind describing the resulting problem?
> > > This is a good question. The symptom is:
> > >
> > > ata_piix: probe of 0000:00:01.1 failed with error -22
> > >
> > >
> > > > With this commit message I'm absolutely not capable to decide whether
> > > > e.g. the other use of nr_legacy_irqs() in pci_xen_initial_domain() is
> > > > correct or not.
> > > I looked at it but I couldn't really test that code because if I try to
> > > change the number of ioapics in the system using the "noapic" command
> > > line option (which actually changes the number if ioapics, not lapics),
> > > I get an error from Linux saying that noapic is not supported when
> > > running on Xen.
> > >
> > > In my opinion having nr_legacy_irqs() calls in Xen code, which returns
> > > 0, is like playing with fire. I think it would be safer/saner to replace
> > > them all with NR_IRQS_LEGACY, simply because reading the code one would
> > > not expect that all those loops don't actually have any iterations.
> > I'm quite sure you should change both uses of nr_legacy_irqs() in
> > pci_xen_initial_domain().
> >
> > Looking at xen_pcifront_enable_irq() I'm not really sure what is the
> > correct thing to do.
> >
> > Adding Konrad as he might have a better insight.
> I wonder if it would be helpful to have a xen-specific #define like
> XEN_NR_LEGACY_PIRQS or something, and document carefully what this means
> and why it is != nr_legacy_irqs().


int xen_nr_legacy_irqs()
{
    if (xen_hvm_domain())
        return nr_legacy_irqs();
    if (xen_initial_domain())
        return NR_IRQS_LEGACY;
    return 0;
}

?

-boris