Re: [Xen-devel] MMIO ioremap() error with PCI passthrough

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 1/7/08 20:24, "Andy Burns" <lists.xensource.com@xxxxxxxxxxxxxx> wrote:

> # dmesg | grep -i apic
> ACPI: APIC CFF80390, 0078 (r1 A_M_I_ OEMAPIC   4000828 MSFT       97)
> ACPI: Local APIC address 0xfee00000
> ACPI: LAPIC (acpi_id[0x01] lapic_id[0x00] enabled)
> ACPI: LAPIC (acpi_id[0x02] lapic_id[0x02] enabled)
> ACPI: LAPIC (acpi_id[0x03] lapic_id[0x01] enabled)
> ACPI: LAPIC (acpi_id[0x04] lapic_id[0x03] enabled)
> ACPI: IOAPIC (id[0x04] address[0xfec00000] gsi_base[0])
> IOAPIC[0]: apic_id 4, address 0xfec00000, GSI 0-23
> ACPI: IOAPIC (id[0x05] address[0xfec10000] gsi_base[24])
> IOAPIC[1]: apic_id 5, address 0xfec10000, GSI 24-279
> Setting APIC routing to xen
> ACPI: Using IOAPIC for interrupt routing
> 
> Is there any way to increase the number of "logical" interrupts to avoid
> (or give the impression of avoiding) sharing? When watching vmware
> servers boot, I remember they split devices into a huge number of
> interrupts numbering 100 upwards.

It would be a lie. Interrupt sharing generally depends on which lines are
wired together on your mainboard. When Xen receives an interrupt on a shared
line it does not have enough information to demultiplex that interrupt to
the appropriate receiver -- it has to broadcast to all potential recipients.
Some of these may regard it as a spurious interrupt because their device did
not raise the interrupt. In our linux-2.6.18-xen tree we specifically handle
this case by disabling spurious irq detection for interrupts which Xen tells
us are shared.

> Can Xen make use of IRQ24 to 279?

I have no idea what IOAPIC[1] above really represents. Usually the GSI range
for an IOAPIC corresponds to its array of interrupt input pins. I've never
heard of a 256-pin IOAPIC!

Anyhow, the assignment of PCI devices to GSIs is determined by mainboard
wiring and firmware configuration. The OS (or hypervisor) does not determine
that relationship.

The best you could hope for would be to try out the MSI/MSI-X support in the
xen-unstable tree (soon to be Xen 3.3). These are message-based interrupts
which completely sidesteps the issue of interrupt lines and their wiring
together.

 -- Keir



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