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

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/22/2010 12:34 PM, Ingo Molnar wrote:
> * Avi Kivity<avi@redhat.com>  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.
>    

Not all KVM vcpus are running operating systems.

Transitive had a product that was using a KVM context to run their 
binary translator which allowed them full access to the host processes 
virtual address space range.  In this case, there is no kernel and there 
are no devices.

That's what I mean by a guest being a userspace context.  KVM simply 
provides a new CPU mode to userspace in the same way that vm8086 mode.

Regards,

Anthony Liguori

>   - 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
>