[libvirt] Node devices?

[ajia]

Ed Swierk wrote:
> After staring at the code implementing node devices in libvirt, I have
> decided that they're really neat, but I have no idea what they are for
> or how to use them.
>
> Reading the API docs and searching libvir-list didn't help much, and
> the user documentation (http://libvirt.org/archnode.html and
> http://libvirt.org/formatnode.html) is a bit, err, high-level.
>
> Can someone enlighten me?
>
> --Ed
>
> --
> libvir-list mailing list
> libvir-list at redhat.com
> https://www.redhat.com/mailman/listinfo/libvir-list
>   
Hi, Ed,
I am not a libvirt developer, maybe I only talk about it with you under 
my understand,
and I hope it is helpful to you.

At first, it will be better if I explain simply libvirt's goal and some 
basic concept:
1.The goal of libvirt: to provide a common generic and stable layer to 
securely manage domains on a node.
2. what is the node?
A node is a single physical machine, in other words, a node is a host 
and a domain is an instance of an
operating system(also known guest)

Node devices should be some devices on the host, such as PCI device, In 
my opinion, you may understand
its function from the use, a group virsh comand can help you, for example:
[root at dhcp-66-70-173 ~]# virsh help | grep node
nodeinfo node information
nodedev-list enumerate devices on this host
nodedev-dumpxml node device details in XML
nodedev-dettach dettach node device its device driver
nodedev-reattach reattach node device its device driver
nodedev-reset reset node device
nodedev-create create a device defined by an XML file on the node
nodedev-destroy destroy a device on the node

1. display host information
[root at dhcp-66-70-173 ~]# virsh nodeinfo
CPU model: x86_64
CPU(s): 2
CPU frequency: 2327 MHz
CPU socket(s): 1
Core(s) per socket: 2
Thread(s) per core: 1
NUMA cell(s): 1
Memory size: 2014320 kB

2. display devices on the host
[root at dhcp-66-70-173 ~]# virsh nodedev-list --tree
computer
|
+-pci_8086_10bd
| |
| +-net_00_1e_4f_db_02_5c
|
+-pci_8086_244e
+-pci_8086_2914
......

3. display specify device xml description
[root at dhcp-66-70-173 ~]# virsh nodedev-dumpxml pci_8086_10bd
<device>
<name>pci_8086_10bd</name>
<parent>computer</parent>
<capability type='pci'>
<domain>0</domain>
<bus>0</bus>
<slot>25</slot>
<function>0</function>
<product id='0x10bd'>82566DM-2 Gigabit Network Connection</product>
<vendor id='0x8086'>Intel Corporation</vendor>
</capability>
</device>

4. dettach host device its device driver
[root at dhcp-66-70-173 ~]# virsh nodedev-dettach pci_8086_10bd
Device pci_8086_10bd dettached

in fact, the operation hide the device host driver and bind the device 
to pci-stub driver
[root at dhcp-66-70-173 ~]# readlink /sys/bus/pci/devices/0000\:00\:19.0/driver
../../../bus/pci/drivers/pci-stub

5. in order to return the device a certain&clean status, we need to 
reset the device
[root at dhcp-66-70-173 ~]# virsh nodedev-reset pci_8086_10bd
Device pci_8086_10bd reset

6. return the device from guest to host
[root at dhcp-66-70-173 ~]# virsh nodedev-reattach pci_8086_10bd
Device pci_8086_10bd re-attached

 From step 2 to step 6, this group operations is very useful, such as we 
pass through a host pci device to guest.

Best Regards,
Alex Jia