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    SubjectRe: [kvm-devel] 2.6.23.1-rt4 and kvm 48
    Carsten Emde wrote:
    > Avi Kivity wrote:
    >> David Brown wrote:
    >>> I thought I'd try out the realtime patch set and it didn't work at all
    >>> with kvm. The console didn't dump anything and the system completely
    >>> locked up.
    > Up to now, the unmodified kvm module never worked with any RT kernel.
    > This would only change, if RT patched kernels were routinely used in the
    > kvm release tests which is, for the time being, not the case.

    As far as I understood (but didn't re-test yet), latest kvm release does
    include all features required to run over -rt. This was at least the
    case for kvm-git I picked up last week and merged into 2.6.23-rt1. The
    only issue is that out-of-tree kvm doesn't enable the preemption
    notifiers (maybe there were one or two further minor issues, I think to
    recall missing smp_ops exports being one, but these were not -rt related).

    >
    > However, tglx once provided a hand-crafted kvm version that can be used
    > together with a selected RT kernel -> see "Latest Stable"
    > (http://www.osadl.org/Latest-Stable.latest-stable-realtime-kernel.0.html)
    > realtime kernel. It is based on kernel 2.6.21.6-rt21 and kvm-28.
    >
    > We used these modules to carry out a number of "proof of concept" tests
    > which were very successful. The realtime capabilities of the host system
    > remained unchanged irrespective of whether the kvm guest system (no
    > graphics) was running or not. This was even the case while the guest
    > system was booting.

    Interesting result - you've read about the wbinvd issues? Is there no
    wbinvd in the bios shipped with older kvm? Which VM extension did you
    test, both Intel and AMD? I would bet that your X issues are due to the
    same effect. X startup/shutdown involves a lot of wbinvd calls on my
    test boxes.

    To sum up my findings: kvm over whatever RT kernel is risky
    (latency-wise) unless you can audit or even para-virtualise your guest
    OS /wrt wbinvd. Maybe we can discuss this also in Linz by the end of the
    week. :)

    Jan

    --
    Siemens AG, Corporate Technology, CT SE 2
    Corporate Competence Center Embedded Linux
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