it seems that with in-kernel irqchip the interrupts are distributed across all vpcus. without in-kernel irqchip all interrupts are on cpu0. maybe this is related.
without inkernel irqchip
CPU0 CPU1 CPU2 CPU3
0: 16 0 0 0 IO-APIC-edge timer
1: 23 0 0 0 IO-APIC-edge i8042
4: 1 0 0 0 IO-APIC-edge
6: 4 0 0 0 IO-APIC-edge floppy
7: 0 0 0 0 IO-APIC-edge parport0
8: 0 0 0 0 IO-APIC-edge rtc0
9: 0 0 0 0 IO-APIC-fasteoi acpi
11: 76 0 0 0 IO-APIC-fasteoi
uhci_hcd:usb1
12: 102 0 0 0 IO-APIC-edge i8042
14: 0 0 0 0 IO-APIC-edge ata_piix
15: 16881 0 0 0 IO-APIC-edge ata_piix
24: 0 0 0 0 PCI-MSI-edge
virtio1-config
25: 5225 0 0 0 PCI-MSI-edge
virtio1-requests
26: 0 0 0 0 PCI-MSI-edge
virtio0-config
27: 72493 0 0 0 PCI-MSI-edge
virtio0-input
...
with inkernel irqchip
CPU0 CPU1 CPU2 CPU3
0: 16 0 0 0 IO-APIC-edge timer
1: 0 3 3 1 IO-APIC-edge i8042
4: 0 0 1 0 IO-APIC-edge
6: 1 0 1 2 IO-APIC-edge floppy
7: 0 0 0 0 IO-APIC-edge parport0
8: 0 0 0 0 IO-APIC-edge rtc0
9: 0 0 0 0 IO-APIC-fasteoi acpi
11: 7 9 4 1 IO-APIC-fasteoi
uhci_hcd:usb1
12: 30 27 29 34 IO-APIC-edge i8042
14: 0 0 0 0 IO-APIC-edge ata_piix
15: 943 937 950 943 IO-APIC-edge ata_piix
24: 0 0 0 0 PCI-MSI-edge
virtio0-config
25: 930 978 980 947 PCI-MSI-edge
virtio0-input
26: 0 0 1 0 PCI-MSI-edge
virtio0-output
27: 0 0 0 0 PCI-MSI-edge
virtio1-config
28: 543 541 542 553 PCI-MSI-edge
virtio1-requests
...
