Re: [PATCH] Fix when booting Xen+Linux under QEMU.

[Stefano Stabellini]

On Thu, 9 Dec 2010, Jeremy Fitzhardinge wrote:
>    2. kill the ioapic dummy page hack (I'm assuming that would be
>       trivial - at least to make it all zero - because the kernel won't
>       care about the number of GSIs at that point)
 
That wouldn't work: mp_register_ioapic reads the number of entries for
each ioapic and it cannot be zero, because later on mp_find_ioapic is
going to find the ioapic a particular gsi belongs to based on the
previous results.
Mp_register_ioapic also increases gsi_top that ends up influencing the
value of nr_irqs_gsi.


Considering that nr_irqs is not a real upper limit on Xen because you
usually have at least 1024 evtchns anyway, I think we should reimplement
arch_probe_nr_irqs on Xen to set nr_irqs appropriately.
Something like this:

#define NR_EVTCHNS (sizeof(unsigned long) * 8 * sizeof(unsigned long))
int __init arch_probe_nr_irqs(void)
{
	if (xen_domain())
		nr_irqs = NR_EVTCHNS;
    
    return NR_IRQS_LEGACY;
}

The problem is that #ifdef CONFIG_SPARSE_IRQ io_apic.c reimplements
arch_probe_nr_irqs too but only if CONFIG_PCI_MSI is also defined
nr_irqs is set high enough by that function.
So to work around this problem we might have to redefine
arch_probe_nr_irqs only #if !defined(CONFIG_SPARSE_IRQ) and we might also
have to automatically enable CONFIG_PCI_MSI if the user selects
CONFIG_SPARSE_IRQ on XEN somehow.
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