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Re: Is it possible to control irq allocation in QEMU?

From: Jakob Bohm
Subject: Re: Is it possible to control irq allocation in QEMU?
Date: Sun, 6 Dec 2020 17:17:23 +0100
User-agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1

On 06/12/2020 10:45, Hao Lee wrote:

I use the following options to simulate USB devices.

-drive if=none,id=usbstick,file=usb.qcow2 \
-usb -device usb-ehci,id=ehci  \
-device usb-tablet,bus=usb-bus.0 \
-device usb-storage,bus=ehci.0,drive=usbstick

The contents in /proc/interrupts are:

   0:        515   IO-APIC   2-edge      timer
   1:         63   IO-APIC   1-edge      i8042
   8:          1   IO-APIC   8-edge      rtc0
   9:          0   IO-APIC   9-fasteoi   acpi
  10:        127   IO-APIC  10-fasteoi   ehci_hcd:usb1
  11:         36   IO-APIC  11-fasteoi   uhci_hcd:usb2
  12:          5   IO-APIC  12-edge      i8042
  14:         93   IO-APIC  14-edge      ata_piix
  15:          6   IO-APIC  15-edge      ata_piix

I want to know why QEMU allocates irq-10 and irq-11 for the two USB devices.
Can I make them above irq-15? Thanks!
The classic PC architecture, as introduced by IBM PC/AT only has the 15 interrupt

The modern IO-APIC may have options to route PCI(e) interrupts to other x86 interrupt
vectors, if activated by the operating system, in this case Linux.

If QEMU emulates such an IO-APIC ability, Linux may have boot options to use it, or it
might be configurable in the emulated SeaBIOS.


Jakob Bohm, CIO, Partner, WiseMo A/S.  http://www.wisemo.com
Transformervej 29, 2860 Soborg, Denmark.  Direct +45 31 13 16 10
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WiseMo - Remote Service Management for PCs, Phones and Embedded

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