[libvirt] [RFC PATCH] support multifunction PCI device

[Wen Congyang]

At 05/10/2011 06:00 PM, Daniel P. Berrange Write:
> On Tue, May 10, 2011 at 02:00:17PM +0800, Wen Congyang wrote:
>> We want to use more than 200+ device. Libvirt does not use multi
>> function PCI device and PCI-to-PCI bridge. So we can not use more
>> than 200+ device if it's a PCI device or it's controller is a PCI
>> device.
> 
> IMHO using multi function support is a dead end, because it
> makes hotplug completely unusable and is not even possible
> for most PCI devices. The way we want to raise the device
> limit is
> 
>   a. Supporting multiple PCI domains (multiple PCI root complexes)

IIUC, qemu do not support multiple PCI domains now.

>   b. Adding PCI bridges

I do not know whether qemu supports it. I find hw/pci_bridge.c in qemu's source,
so qemu may support it. I will confirm it.

>   c. A virtio-scsi controller to allow > 1 virtio disk per PCI device

Hmm, do you mean this:
-device virtio-blk-pci,ports=4,drive0=hda,drive1=hdb,drive2=hdc,...

> 
> any of those will dramatically increase the number of devices
> we can use without the horrible hotplug problems that multifunction
> introduces.
> 
>> This patch adds the support of multi function PCI device. It
>> does not support to hot plug/unplug multi function PCI device.
>>
>> TODO:
>> 1. support to hot plug multi function PCI device
>>    We only can hot plug one device at one time. I think we should
>>    introduce another command 'virsh attach-devices XXX' to support
>>    hot plug more than one device at one time?
> 
> Since you can't do practical hotplug of multifunction devices at the
> QEMU layer at all, there's nothing useful we can do at libvirt either.

We can hotplug multifunction PCI devices like this:
1. Before hot pluging,
# lspci
00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
00:01.2 USB Controller: Intel Corporation 82371SB PIIX3 USB [Natoma/Triton II] (rev 01)
00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)
00:02.0 VGA compatible controller: Cirrus Logic GD 5446
00:03.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 20)
00:04.0 RAM memory: Red Hat, Inc Virtio memory balloon
00:05.0 SCSI storage controller: LSI Logic / Symbios Logic 53c895a

2. hot plug multi function PCI devices:
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi2,bus=pci.0,addr=0x06.0x07'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi3,bus=pci.0,addr=0x06.0x06'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi4,bus=pci.0,addr=0x06.0x05'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi5,bus=pci.0,addr=0x06.0x04'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi6,bus=pci.0,addr=0x06.0x03'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi7,bus=pci.0,addr=0x06.0x02'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi8,bus=pci.0,addr=0x06.0x01'
# virsh qemu-monitor-command vm1 --hmp 'device_add lsi,id=scsi9,multifunction=on,bus=pci.0,addr=0x06.0x00'

3. After hot plug multi function PCI devices:
# lspci
00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
00:01.2 USB Controller: Intel Corporation 82371SB PIIX3 USB [Natoma/Triton II] (rev 01)
00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)
00:02.0 VGA compatible controller: Cirrus Logic GD 5446
00:03.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 20)
00:04.0 RAM memory: Red Hat, Inc Virtio memory balloon
00:05.0 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.0 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.1 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.2 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.3 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.4 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.5 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.6 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
00:06.7 SCSI storage controller: LSI Logic / Symbios Logic 53c895a


> 
>>
>> 2. support to hot unplug multi function PCI device
>>    hot unpluging multi function PCI device meas that all the other
>>    devices on the same slot will be hot unpluged. So we should do
>>    some cleanup and remove the other devices too. If the other
>>    device on the same slot does not support hot unpluged, or it is a
>>    a controller and some other devices still use this controller,
>>    I think we should refuse to hot unplug this mutlti function PCI
>>    device.
> 
> IMHO these kind of restrictions will make life really unpleasant
> for applications and are a reason we should *not* support the
> multifunction code. Instead we should focus on one of the other
> 3 options I mention above.

Yes, there's too many restrictions about hot plug/unplug multifunction
PCI devices.

> 
> Daniel