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

[Avi Kivity <avi@redhat.com>]

On 03/24/2010 02:50 PM, Joerg Roedel wrote:
>
>> You can always provide the kernel and module paths as command line
>> parameters.  It just won't be transparently usable, but if you're using
>> qemu from the command line, presumably you can live with that.
>>      
> I don't want the tool for myself only. A typical perf user expects that
> it works transparent.
>    

A typical kvm user uses libvirt, so we can integrate it with that.

>>> Could be easily done using notifier chains already in the kernel.
>>> Probably implemented with much less than 100 lines of additional code.
>>>        
>> And a userspace interface for that.
>>      
> Not necessarily. The perf event is configured to measure systemwide kvm
> by userspace. The kernel side of perf takes care that it stays
> system-wide even with added vm instances. So in this case the consumer
> for the notifier would be the perf kernel part. No userspace interface
> required.
>    

Someone needs to know about the new guest to fetch its symbols.  Or do 
you want that part in the kernel too?

>> If we make an API, I'd like it to be generally useful.
>>      
> Thats hard to do at this point since we don't know what people will use
> it for. We should keep it simple in the beginning and add new features
> as they are requested and make sense in this context.
>    

IMO this use case is to rare to warrant its own API, especially as there 
are alternatives.

>> It's a total headache.  For example, we'd need security module hooks to
>> determine access permissions.  So far we managed to avoid that since kvm
>> doesn't allow you to access any information beyond what you provided it
>> directly.
>>      
> Depends on how it is designed. A filesystem approach was already
> mentioned. We could create /sys/kvm/ for example to expose information
> about virtual machines to userspace. This would not require any new
> security hooks.
>    

Who would set the security context on those files?  Plus, we need cgroup 
support so you can't see one container's guests from an unrelated container.

>> Copying the objects is a one time cost.  If you run perf for more than a
>> second or two, it would fetch and cache all of the data.  It's really
>> the same problem with non-guest profiling, only magnified a bit.
>>      
> I don't think we can cache filesystem data of a running guest on the
> host. It is too hard to keep such a cache coherent.
>    

I don't see any choice.  The guest can change its symbols at any time 
(say by kexec), without any notification.

>>>> Other userspaces can also provide this functionality, like they have to
>>>> provide disk, network, and display emulation.  The kernel is not a huge
>>>> library.
>>>>          
> If two userspaces run in parallel what is the single instance where perf
> can get a list of guests from?
>    

I don't know.  Surely that's solvable though.

>> kvm.ko has only a small subset of the information that is used to define
>> a guest.
>>      
> The subset is not small. It contains all guest vcpus, the complete
> interrupt routing hardware emulation and manages event the guests
> memory.
>    

It doesn't contain most of the mmio and pio address space.  Integration 
with qemu would allow perf to tell us that the guest is hitting the 
interrupt status register of a virtio-blk device in pci slot 5 (the 
information is already available through the kvm_mmio trace event, but 
only qemu can decode it).

-- 
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