[compsciphd]

I actually wonder how much "overhead" a VM actually has. i.e. a linux kernel that doesn't do anything (say perhaps just boots to an init that mounts proc and every n seconds read in/prints out /proc/meminfo) how much memory would the kernel actually be using?

So if processes in gvisor map to processes on the underlying kernel, I'd agree it gives one a better ability to introspect (at least in an easy manner).

It gives me an idea that I'd think would be interesting (I think this has been done, but it escapes me where), to have a tool that is external to the VM (runs on the hypervisor host) that essentially has "read only" access to the kernel running in the VM to provide visibility into what's running on the machine without an agent running within the VM itself. i.e. something that knows where the processes list is, and can walk it to enumerate what's running on the system.

I can imagine the difficulties in implementing such a thing (especially on a multi cpu VM), where even if you could snapshot the kernel memory state efficiently, it be difficult to do it in a manner that provided a "safe/consistent" view. It might be interesting if the kernel itself could make a hypercall into the hypervisor at points of consistency (say when finished making an update and about to unlock the resource) to tell the tool when the data can be collected.

[stacktrust]

https://github.com/Wenzel/pyvmidbg

  LibVMI-based debug server, implemented in Python. Building a guest aware, stealth and agentless full-system debugger.. GDB stub allows you to debug a remote process running in a VM with your favorite GDB frontend. By leveraging virtual machine introspection, the stub remains stealth and requires no modification of the guest.

more: https://github.com/topics/virtual-machine-introspection

[compsciphd]

thanks, the kvm-vmi is basically an expansive version of what I was imagining (maybe read about it before, as noted, I thought it existed).

[stacktrust]

Would you recommend ZFS as a building block for modern "VLFS"?

https://blog.chlc.cc/p/docker-and-zfs-a-tough-pair/

[eru]

> I actually wonder how much "overhead" a VM actually has. i.e. a linux kernel that doesn't do anything (say perhaps just boots to an init that mounts proc and every n seconds read in/prints out /proc/meminfo) how much memory would the kernel actually be using?

You don't necessarily need to run a full operating system in your VM. See eg https://mirage.io/

[ecnahc515]

> I actually wonder how much "overhead" a VM actually has. i.e. a linux kernel that doesn't do anything (say perhaps just boots to an init that mounts proc and every n seconds read in/prints out /proc/meminfo) how much memory would the kernel actually be using?

There's already some memory sharing available using DAX in Kata Containers at least: https://github.com/kata-containers/kata-containers/blob/main...

[xtacy]

> to have a tool that is external to the VM (runs on the hypervisor host) that essentially has "read only" access to the kernel running on the VM to provide visibility into what's running on the machine without an agent running within the VM itself

Not quite what you are after, but comes close ... you could run gdb on the kernel in this fashion and inspect, pause, step through kernel code: https://stackoverflow.com/questions/11408041/how-to-debug-th....

[XorNot]

What I really want is a "magic" shell on a VM - i.e. the ability using introspection calls to launch a process on the VM which gives me stdin/stdout, and is running bash or something - but is just magically there via an out-of-band mechanism.

[compsciphd]

Not really "out of band", but many VMs allow you to setup a serial console, which is sort of that, albeit with a login, but in reality, could create one without one, still have to go through hypervisor auth to access it in all cases, so perhaps good enough for your case?

[blipvert]

Indeed, easy enough to get a serial device on Xen.

Another possibility could be to implement a simple protocol which uses the xenstore key/value interface to pass messages between host and guest?

[b112]

You can launch KVM/qemu with screen + text console, and just log in there. You can also configure KVM to have a VNC session on launch, and that ... while graphical, is another eye into the console + login.

(Just mentioning two ways without serial console to handle this, although serial console would be fine.)