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

[Stefan Hajnoczi]

On Wed, May 05, 2021 at 04:38:55PM -0400, 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).

The crosvm command-line syntax is: crosvm run --cid <CID>

> 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
> 
>   # ps aux | grep 1954
>   qemu        1954  9.9 21.3 1629092 796148 ?      Sl   16:22   0:58  /usr/bin/qemu-kvm -name guest=Fedora21,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-1-Fedora21/master-key.aes -machine pc-1.2,accel=kvm,usb=off,dump-guest-core=off -cpu qemu64 -m 1000 -overcommit mem-lock=off -smp 2,sockets=2,cores=1,threads=1 -uuid 1eefeeb0-3ac7-07c1-926e-236908313b4c -no-user-config -nodefaults -chardev socket,id=charmonitor,fd=32,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot strict=on -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x6 -blockdev {"driver":"host_device","filename":"/dev/mapper/vg_bxtest-GuestFedora","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"} -blockdev {"node-name":"libvirt-1-format","read-only":false,"driver":"raw","file":"libvirt-1-storage"} -device ide-hd,bus=ide.0,unit=0,drive=libvirt-1-
>  format,id=ide0-0-0,bootindex=1 -netdev tap,fd=34,id=hostnet0 -device rtl8139,netdev=hostnet0,id=net0,mac=52:54:00:9f:e9:d5,bus=pci.0,addr=0x3 -netdev tap,fd=35,id=hostnet1 -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:ec:dc:6e,bus=pci.0,addr=0x5 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev pipe,id=charchannel0,path=/var/lib/trace-cmd/virt/Fedora21/trace-pipe-cpu0 -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=trace-pipe-cpu0 -chardev pipe,id=charchannel1,path=/var/lib/trace-cmd/virt/Fedora21/trace-pipe-cpu1 -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=trace-pipe-cpu1 -vnc 127.0.0.1:0 -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -device vhost-vsock-pci,id=vsock0,guest-cid=3,vhostfd=16,bus=pci.0,addr=0x7 -msg 
>  timestamp=on
>   root        2000  0.0  0.0      0     0 ?        S    16:22   0:00 [kvm-pit/1954]
>   root        2002  0.0  0.0      0     0 ?        S    16:22   0:00 [vhost-1954]

This approach relies on process hierarchy of the VMM (QEMU).
Multi-process QEMU is in development and will allow VIRTIO devices to
run as separate processes from the main QEMU. It then becomes harder to
correlate a VIRTIO device process with its QEMU process.

So I think in the end this approach ends up being as fragile as parsing
command-lines. The kernel doesn't really have the concept of a "VM" that
the vhost_vsock is associated with :). Maybe just parse QEMU and crosvm
command-lines?

Stefan

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