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

Avi Kivity
2010-03-24T13:57:39Z

On 03/24/2010 03:46 PM, Joerg Roedel wrote:
> On Wed, Mar 24, 2010 at 03:05:02PM +0200, Avi Kivity wrote:
>
>> On 03/24/2010 02:50 PM, Joerg Roedel wrote:
>>
>
>>> 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.
>>
> Someone who uses libvirt and virt-manager by default is probably not
> interested in this feature at the same level a kvm developer is. And
> developers tend not to use libvirt for low-level kvm development. A
> number of developers have stated in this thread already that they would
> appreciate a solution for guest enumeration that would not involve
> libvirt.
>

So would I. But when I weigh the benefit of truly transparent
system-wide perf integration for users who don't use libvirt but do use
perf, versus the cost of transforming kvm from a single-process API to a
system-wide API with all the complications that I've listed, it comes
out in favour of not adding the API.

Those few users can probably script something to cover their needs.

>> Someone needs to know about the new guest to fetch its symbols. Or do
>> you want that part in the kernel too?
>>
> The samples will be tagged with the guest-name (and some additional
> information perf needs). Perf userspace can access the symbols then
> through /sys/kvm/guest0/fs/...
>

I take that as a yes? So we need a virtio-serial client in the kernel
(which might be exploitable by a malicious guest if buggy) and a
fs-over-virtio-serial client in the kernel (also exploitable).

>>> 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?
>>
> An approach like: "The files are owned and only readable by the same
> user that started the vm." might be a good start. So a user can measure
> its own guests and root can measure all of them.
>

That's not how sVirt works. sVirt isolates a user's VMs from each
other, so if a guest breaks into qemu it can't break into other guests
owned by the same user.

The users who need this API (!libvirt and perf) probably don't care
about sVirt, but a new API must not break it.

>> Plus, we need cgroup support so you can't see one container's guests
>> from an unrelated container.
>>
> cgroup support is an issue but we can solve that too. Its in general
> still less complex than going through the whole libvirt-qemu-kvm stack.
>

It's a tradeoff. IMO, going through qemu is the better way, and also
provides more information.

>> 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).
>>
> Yeah that would be interesting information. But it is more related to
> tracing than to pmu measurements.
> The information which you mentioned above are probably better
> captured by an extension of trace-events to userspace.
>

It's all related. You start with perf, see a problem with mmio, call up
a histogram of mmio or interrupts or whatever, then zoom in on the
misbehaving device.

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