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

    error compiling committee.c: too many arguments to function

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