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

From: Stefano Garzarella
Date: Fri May 07 2021 - 10:11:35 EST

Hi Steven,

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).

For debug I think could be useful to link the vhost-vsock kthread to the CID, but for the user point of view, maybe is better to query the VM management layer, for example if you're using libvirt, you can easily do:

$ virsh dumpxml fedora34 | grep cid
<cid auto='yes' address='3'/>


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.

I honestly don't know if it's the best interface, like I said maybe for debugging it's fine, but if we want to expose it to the user in some way, we could support devlink/netlink to provide information about the vsock devices currently in use.


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 is just an example of what I'm looking for. Just a way to find what
process is using what cid.

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 5e78fb719602..4f03b25b23c1 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -15,6 +15,7 @@
#include <linux/virtio_vsock.h>
#include <linux/vhost.h>
#include <linux/hashtable.h>
+#include <linux/debugfs.h>

#include <net/af_vsock.h>
#include "vhost.h"
@@ -900,6 +901,128 @@ static struct miscdevice vhost_vsock_misc = {
.fops = &vhost_vsock_fops,
};

+static struct dentry *vsock_file;
+
+struct vsock_file_iter {
+ struct hlist_node *node;
+ int index;
+};
+
+
+static void *vsock_next(struct seq_file *m, void *v, loff_t *pos)
+{
+ struct vsock_file_iter *iter = v;
+ struct vhost_vsock *vsock;
+
+ if (pos)
+ (*pos)++;
+
+ if (iter->index >= (int)HASH_SIZE(vhost_vsock_hash))
+ return NULL;
+
+ if (iter->node)
+ iter->node = rcu_dereference_raw(hlist_next_rcu(iter->node));
+
+ for (;;) {
+ if (iter->node) {
+ vsock = hlist_entry_safe(rcu_dereference_raw(iter->node),
+ struct vhost_vsock, hash);
+ if (vsock->guest_cid)
+ break;
+ iter->node = rcu_dereference_raw(hlist_next_rcu(iter->node));
+ continue;
+ }
+ iter->index++;
+ if (iter->index >= HASH_SIZE(vhost_vsock_hash))
+ return NULL;
+
+ iter->node = rcu_dereference_raw(hlist_first_rcu(&vhost_vsock_hash[iter->index]));
+ }
+ return iter;
+}
+
+static void *vsock_start(struct seq_file *m, loff_t *pos)
+{
+ struct vsock_file_iter *iter = m->private;
+ loff_t l = 0;
+ void *t;
+
+ rcu_read_lock();

Instead of keeping this rcu lock between vsock_start() and vsock_stop(), maybe it's better to make a dump here of the bindings (pid/cid), save it in an array, and iterate it in vsock_next().

+
+ iter->index = -1;
+ iter->node = NULL;
+ t = vsock_next(m, iter, NULL);
+
+ for (; iter->index < HASH_SIZE(vhost_vsock_hash) && l < *pos;
+ t = vsock_next(m, iter, &l))
+ ;

A while() maybe was more readable...

Thanks,
Stefano