Hotadd pci devices

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Using PCI Hotplug Support

PCI Hotplug support is now available in kvm.git, here is how to use it.

Install PCI Hotplug drivers in the guest (Linux)

I loaded the following modules to get pci hotplug working:

  • acpiphp
  • pci_hotplug

You can either add these to your distro's module list to load on boot, or run a command like this. 

for m in acpiphp pci_hotplug; do sudo modprobe ${m}; done

Use QEMU monitor to hot add device

(qemu) drive_add 0 file=/storage/10G.img,format=qcow2,id=drive-virtio-disk1,if=none 
(qemu) device_add virtio-blk-pci,scsi=on,drive=drive-virtio-disk1
  • check attached PCI devices
(qemu) info pci
  • notice
    • pci_add is old style, instead use device_add

Use QEMU monitor to hot add devices

The QEMU monitor has a new command, pci_add, which is used to hotadd the pci devices. 


QEMU 0.9.1 monitor - type 'help' for more information
(qemu) help pci_add 
pci_add bus nic|storage [[vlan=n][,macaddr=addr][,model=type]] [file=file][,if=type][,bus=nr]... -- hot-add PCI device


Add a NIC

The following command in the QEMU monitor hot-adds a nic to the guest.

  • pci_add auto nic model=e1000

You can replace model=e1000 with either model=rtl8139, model=virtio, or model=ne2k_pci. Note that model=ne2000 isn't pci-based but rather ISA-based and cannot be used for hotplug. 

(qemu) pci_add auto nic  model=e1000 WARNING: PCI region size must be pow2 type=0x0, size=0x60000  OK bus 0, slot 5, function 0 (devfn 40)

In the guest we can check if the device was added by looking at dmesg. 

[   91.808265] acpiphp: Slot [30] registered                          
[   91.808276] acpiphp: Slot [31] registered                          
[  110.828130] PCI: Ignore bogus resource 6 [0:0] of 0000:00:02.0     
[  110.828333] decode_hpp: Could not get hotplug parameters. Use defaults
[  110.903259] Intel(R) PRO/1000 Network Driver - version 7.3.20-k2-NAPI 
[  110.903264] Copyright (c) 1999-2006 Intel Corporation.                
[  110.903489] ACPI: PCI Interrupt Link [LNKA] enabled at IRQ 10         
[  110.903492] ACPI: PCI Interrupt 0000:00:05.0[A] -> Link [LNKA] -> GSI 10 (level, high) -> IRQ 10
[  111.238713] e1000: 0000:00:05.0: e1000_probe: (PCI:33MHz:32-bit) 52:54:00:12:34:57              
[  111.292550] e1000: eth1: e1000_probe: Intel(R) PRO/1000 Network Connection


Add a disk

Hot-adding a disk means hot-adding a PCI device, so you need PCI hotplug support in the guest for this to work.

The following command in the QEMU monitor will add a scsi disk to the guest: 

 * pci_add auto storage file=/home/rharper/work/images/empty.img,if=scsi

You can replace if=scsi with if=virtio, but IDE is not hotpluggable. 


QEMU 0.9.1 monitor - type 'help' for more information

(qemu) pci_add auto storage file=/home/rharper/work/images/empty.img,if=scsi 

OK bus 0, slot 5, function 0 (devfn 40)

In the guest we see via dmesg that the device was added. 

[  150.581752] PCI: Enabling device 0000:00:05.0 (0000 -> 0003)
[  150.582634] ACPI: PCI Interrupt Link [LNKA] enabled at IRQ 10
[  150.582637] ACPI: PCI Interrupt 0000:00:05.0[A] -> Link [LNKA] -> GSI 10 (level, high) -> IRQ 10
[  150.585099] sym0: <895a> rev 0x0 at pci 0000:00:05.0 irq 10
[  150.594628] sym0: No NVRAM, ID 7, Fast-40, LVD, parity checking
[  150.607861] sym0: SCSI BUS has been reset.
[  150.607883] scsi2 : sym-2.2.3
[  153.608433] scsi 2:0:0:0: Direct-Access     QEMU     QEMU HARDDISK    0.9. PQ: 0 ANSI: 3                                                     
[  153.608462]  target2:0:0: tagged command queuingenabled, command queue depth 16. 
[  153.608484]  target2:0:0: Beginning Domain Validation
[  153.609517]  target2:0:0: Domain Validation skipping write tests
[  153.609522]  target2:0:0: Ending Domain Validation
[  153.618880] scsi 2:0:0:0: Attached scsi generic sg1 type 0
[  153.749967] Driver 'sd' needs updating - please use bus_type methods
[  153.750204] sd 2:0:0:0: [sda] 20971520 512-byte hardware sectors (10737 MB)
[  153.750253] sd 2:0:0:0: [sda] Write Protect is off
[  153.750255] sd 2:0:0:0: [sda] Mode Sense: 14 00 00 00
[  153.750354] sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[  153.750478] sd 2:0:0:0: [sda] 20971520 512-byte hardware sectors (10737 MB)
[  153.750253] sd 2:0:0:0: [sda] Write Protect is off
[  153.750255] sd 2:0:0:0: [sda] Mode Sense: 14 00 00 00
[  153.750354] sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[  153.750478] sd 2:0:0:0: [sda] 20971520 512-byte hardware sectors (10737 MB)
[  153.750534] sd 2:0:0:0: [sda] Write Protect is off
[  153.750536] sd 2:0:0:0: [sda] Mode Sense: 14 00 00 00
[  153.750634] sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[  153.750637]  sda: unknown partition table
[  153.759275] sd 2:0:0:0: [sda] Attached SCSI disk
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