Re: Instructions for clock sync for tracing host/guest

[Dario Faggioli <dfaggioli@suse.com>]

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On Mon, 2021-04-26 at 11:50 -0400, Steven Rostedt wrote:
> On Mon, 26 Apr 2021 17:38:09 +0200
> Dario Faggioli <dfaggioli@suse.com> wrote:
> > 
> > With the PID of the creating process you can tell (e.g., still from
> > /proc) the number of the vm-fd, and we have everything to reach TSC
> > offsets in debugfs... Isn't that so?
> 
> I'm not sure what you mean here.
> 
So:

ps -eT |grep "CPU 0/KVM"
  PID  SPID TTY          TIME CMD
 7037  7050 ?        00:26:31 CPU 0/KVM

That's vCPU 0's host task. It's a thread whose host PID is 7050. With
trace_cmd, you'll see kvm_enter events happening in task 7050, so
that's indeed a really good way to retrieve such PID.

The process that created this vCPU thread is 7037, which in my case is
in fact QEMU:

ps aux |grep 7037
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
qemu      7037  336 17.8 63578168 11647320 ?   Sl   Apr26 1489:18 /usr/bin/qemu-system-x86_64 -name guest=vm-kvm...

This QEMU process (7037) is the one that created the VM itself, with
the appropriate ioctl, which returned to it the VM-fd:

ls -lR /proc/7037/fd/*|grep kvm-vm
lrwx------ 1 qemu qemu 64 Apr 26 22:01 /proc/7037/fd/14 -> anon_inode:kvm-vm

So, in this case, it's fd 14. And, in fact, in debugfs, we have:

ls /sys/kernel/debug/kvm/ -l                                          
drwxr-xr-x 34 root root 0 Apr 26 16:41 7037-14

Hope is clearer (and ideally even useful :-P)

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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