virtual CPU in KVM - linux

Question

Does anybody know, where vcpu thread id is stored in linux? In fact according to my researches when we create a VM in KVM, some threads will shape vcpus; i need the ID and location of them.
I took a look at this position:
/proc/qemu-kvm process ID/tasks/*/
the qemu-kvm process ID field comes from this location:
/var/run/libvirt/qemu/VM_NAME.xml
BECAUSE i thought that maybe vcpu's ID could be found there, but unfortunately they are not vcpu's ID, they are just some child process.
any help would be appreciated.
thanks a lot.

Answer 1

If you exec qemu with parameters -qmp unix:./qmp-sock,server,nowait, for example:
# /opt/qemu/bin/qemu-system-x86_64 \
-smp cpus=2 \
-drive file=/opt/test.qcow2,format=qcow2 \
-cdrom CentOS-7-x86_64-DVD-1511.iso \
-qmp unix:./qmp-sock,server,nowait
You can exec qmp-shell to get cpu info:
# /opt/git/qemu/scripts/qmp/qmp-shell /opt/qmp-sock
Welcome to the QMP low-level shell!
Connected to QEMU 2.5.50
(QEMU) query-cpus
{"return": [{"halted": false, "pc": -2124176787, "current": true, "qom_path": "/machine/unattached/device[0]", "thread_id": 2344, "arch": "x86", "CPU": 0}, {"halted": true, "pc": -2130342250, "current": false, "qom_path": "/machine/unattached/device[3]", "thread_id": 2341, "arch": "x86", "CPU": 1}]}
Thread id here: 2344 and 2341
# ps -eLf|grep qemu-system
root 2341 2252 2341 9 4 08:52 pts/0 00:00:48 /opt/qemu/bin/qemu-system-x86_64 -smp cpus=2 -drive file=/opt/test.qcow2,format=qcow2 -cdrom CentOS-7-x86_64-DVD-1511.iso -qmp unix:./qmp-sock,server,nowait
root 2341 2252 2342 0 4 08:52 pts/0 00:00:00 /opt/qemu/bin/qemu-system-x86_64 -smp cpus=2 -drive file=/opt/test.qcow2,format=qcow2 -cdrom CentOS-7-x86_64-DVD-1511.iso -qmp unix:./qmp-sock,server,nowait
root 2341 2252 2344 85 4 08:52 pts/0 00:07:04 /opt/qemu/bin/qemu-system-x86_64 -smp cpus=2 -drive file=/opt/test.qcow2,format=qcow2 -cdrom CentOS-7-x86_64-DVD-1511.iso -qmp unix:./qmp-sock,server,nowait
root 2341 2252 2345 0 4 08:52 pts/0 00:00:00 /opt/qemu/bin/qemu-system-x86_64 -smp cpus=2 -drive file=/opt/test.qcow2,format=qcow2 -cdrom CentOS-7-x86_64-DVD-1511.iso -qmp unix:./qmp-sock,server,nowait
root 2378 2304 2378 0 1 09:01 pts/2 00:00:00 grep --color=auto qemu-system
For more information see http://wiki.qemu.org/QMP

Answer 2

I think vcpu thread ID is internal to Qemu and it is exposed to linux as a normal thread
struct CPUState {
...
struct QemuThread *thread;
...
int thread_id;
...
bool thread_kicked;
...
bool throttle_thread_scheduled;
...
};
You can use Qemu command info cpus to show information about cpus. It gives me this:
(qemu) info cpus
* CPU #0: pc=0x00000000b483c8c4 thread_id=6660