Re: [RFC][PATCH] vhost/vsock: Add vsock_list file to map cid with vhost tasks

[Mike Christie]

On 5/5/21 3:38 PM, Steven Rostedt wrote:
> The new trace-cmd 3.0 (which is almost ready to be released) allows for
> tracing between host and guests with timestamp synchronization such that
> the events on the host and the guest can be interleaved in the proper order
> that they occur. KernelShark now has a plugin that visualizes this
> interaction.
> 
> The implementation requires that the guest has a vsock CID assigned, and on
> the guest a "trace-cmd agent" is running, that will listen on a port for
> the CID. The on the host a "trace-cmd record -A guest@cid:port -e events"
> can be called and the host will connect to the guest agent through the
> cid/port pair and have the agent enable tracing on behalf of the host and
> send the trace data back down to it.
> 
> The problem is that there is no sure fire way to find the CID for a guest.
> Currently, the user must know the cid, or we have a hack that looks for the
> qemu process and parses the --guest-cid parameter from it. But this is
> prone to error and does not work on other implementation (was told that
> crosvm does not use qemu).
> 
> As I can not find a way to discover CIDs assigned to guests via any kernel
> interface, I decided to create this one. Note, I'm not attached to it. If
> there's a better way to do this, I would love to have it. But since I'm not
> an expert in the networking layer nor virtio, I decided to stick to what I
> know and add a debugfs interface that simply lists all the registered CIDs
> and the worker task that they are associated with. The worker task at
> least has the PID of the task it represents.
> 
> Now I can find the cid / host process in charge of the guest pair:
> 
>   # cat /sys/kernel/debug/vsock_list
>   3	vhost-1954:2002
> 

I think I need the same thing for vhost-scsi. We want to know a vhost-scsi
devs worker thread's pid. If we use multiple vhost-devs in one VM then we
wanted to be able to know which thread goes with which dev.

For the vhost thread patches I added an ioctl:

https://lists.linuxfoundation.org/pipermail/virtualization/2021-April/054014.html

but I had originally implemented it in sysfs. For sysfs we can add a struct
device in the vhost_dev and struct deice in the vhost_virtqueue. We then
have a 2 new classes /sys/class/vhost_device and vhost_virtqueue with the
vhost_device device the parent of vhost_virtqueue device.

The nice thing is that it's a common interface and works for every vhost_dev
and all their virtqueues. It works for non libvirt users.

The drawback is adding in refcounts/releases and that type of code for the
vhost_dev and vhost_virtqueue. Also I'm not sure about security.

Note that I'm not tied to sysfs. netlink would be fine. I just need any
interface.