Re: [RFC] Unify KVM kernel-space and user-space code into a singleproject

[Ingo Molnar]

* Avi Kivity <avi@xxxxxxxxxx> wrote:

> >>>  - Easy default reference to guest instances, and a way for tools to
> >>>    reference them symbolically as well in the multi-guest case. Preferably
> >>>    something trustable and kernel-provided - not some indirect information
> >>>    like a PID file created by libvirt-manager or so.
> >>
> >> Usually 'layering violation' is trotted out at such suggestions.
> >> [...]
> >
> > That's weird, how can a feature request be a 'layering violation'?
> 
> The 'something trustable and kernel-provided'.  The kernel knows nothing 
> about guest names.

The kernel certainly knows about other resources such as task names or network 
interface names or tracepoint names. This is kernel design 101.

> > If something that users find straightforward and usable is a layering 
> > violation to you (such as easily being able to access their own files on 
> > the host as well ...) then i think you need to revisit the definition of 
> > that term instead of trying to fix the user.
> 
> Here is the explanation, you left it quoted:
> 
> >> [...]  I don't like using the term, because sometimes the layers are 
> >> incorrect and need to be violated.  But it should be done explicitly, not 
> >> as a shortcut for a minor feature (and profiling is a minor feature, most 
> >> users will never use it, especially guest-from-host).
> >>
> >> The fact is we have well defined layers today, kvm virtualizes the cpu 
> >> and memory, qemu emulates devices for a single guest, libvirt manages 
> >> guests. We break this sometimes but there has to be a good reason.  So 
> >> perf needs to talk to libvirt if it wants names.  Could be done via 
> >> linking, or can be done using a pluging libvirt drops into perf.

This is really just the much-discredited microkernel approach for keeping 
global enumeration data that should be kept by the kernel ...

Lets look at the ${HOME}/.qemu/qmp/ enumeration method suggested by Anthony. 
There's numerous ways that this can break:

 - Those special files can get corrupted, mis-setup, get out of sync, or can
   be hard to discover.

 - The ${HOME}/.qemu/qmp/ solution suggested by Anthony has a very obvious
   design flaw: it is per user. When i'm root i'd like to query _all_ current
   guest images, not just the ones started by root. A system might not even
   have a notion of '${HOME}'.

 - Apps might start KVM vcpu instances without adhering to the
   ${HOME}/.qemu/qmp/ access method.

 - There is no guarantee for the Qemu process to reply to a request - while
   the kernel can always guarantee an enumeration result. I dont want 'perf 
   kvm' to hang or misbehave just because Qemu has hung.

Really, for such reasons user-space is pretty poor at doing system-wide 
enumeration and resource management. Microkernels lost for a reason.

You are committing several grave design mistakes here.

Thanks,

	Ingo
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