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    SubjectRe: [PATCH 0/2 v3] kvm: notify host when guest panicked
    At 03/14/2012 06:52 PM, Gleb Natapov Wrote:
    > On Wed, Mar 14, 2012 at 06:52:07PM +0800, Wen Congyang wrote:
    >> At 03/14/2012 06:37 PM, Amit Shah Wrote:
    >>> On (Wed) 14 Mar 2012 [17:53:00], Wen Congyang wrote:
    >>>> At 03/14/2012 05:24 PM, Avi Kivity Wrote:
    >>>>> On 03/14/2012 10:29 AM, Wen Congyang wrote:
    >>>>>> At 03/13/2012 06:47 PM, Avi Kivity Wrote:
    >>>>>>> On 03/13/2012 11:18 AM, Daniel P. Berrange wrote:
    >>>>>>>> On Mon, Mar 12, 2012 at 12:33:33PM +0200, Avi Kivity wrote:
    >>>>>>>>> On 03/12/2012 11:04 AM, Wen Congyang wrote:
    >>>>>>>>>> Do you have any other comments about this patch?
    >>>>>>>>> Not really, but I'm not 100% convinced the patch is worthwhile. It's
    >>>>>>>>> likely to only be used by Linux, which has kexec facilities, and you can
    >>>>>>>>> put talk to management via virtio-serial and describe the crash in more
    >>>>>>>>> details than a simple hypercall.
    >>>>>>>> As mentioned before, I don't think virtio-serial is a good fit for this.
    >>>>>>>> We want something that is simple & guaranteed always available. Using
    >>>>>>>> virtio-serial requires significant setup work on both the host and guest.
    >>>>>>> So what? It needs to be done anyway for the guest agent.
    >>>>>>>> Many management application won't know to make a vioserial device available
    >>>>>>>> to all guests they create.
    >>>>>>> Then they won't know to deal with the panic event either.
    >>>>>>>> Most administrators won't even configure kexec,
    >>>>>>>> let alone virtio serial on top of it.
    >>>>>>> It should be done by the OS vendor, not the individual admin.
    >>>>>>>> The hypercall requires zero host
    >>>>>>>> side config, and zero guest side config, which IMHO is what we need for
    >>>>>>>> this feature.
    >>>>>>> If it was this one feature, yes. But we keep getting more and more
    >>>>>>> features like that and we bloat the hypervisor. There's a reason we
    >>>>>>> have a host-to-guest channel, we should use it.
    >>>>>> I donot know how to use virtio-serial.
    >>>>> I don't either, copying Amit.
    >>>>>> I start vm like this:
    >>>>>> qemu ...\
    >>>>>> -device virtio-serial \
    >>>>>> -chardev socket,path=/tmp/foo,server,nowait,id=foo \
    >>>>>> -device virtserialport,chardev=foo,name=port1 ...
    >>>>>> You said that there are too many channels. Does it mean /tmp/foo is a channel?
    >>>>> Probably.
    >>>> Hmm, if we use virtio-serial, the guest kernel writes something into the channel when
    >>>> the os is panicked. Is it right?
    >>> Depends on how you want to use it. It could be the kernel, or it
    >>> could be a userspace program which monitors syslogs for panic
    >>> information and passes on that info to the virtio-serial channel.
    >> When the kernel is panicked, we cannot use userspace program.
    >>>> If so, is this channel visible to guest userspace? If the channle is visible to guest
    >>>> userspace, the program running in userspace may write the same message to the channel.
    >>> Access control is via permissions. You can have udev scripts assign
    >>> whatever uid and gid to the port of your interest. By default, all
    >>> ports are only accessible to the root user.
    >> We should also prevent root user writing message to this channel if it is
    >> used for panicked notification.
    > Why? Root user can also call panic hypercall if he wishes so.

    IIRC, the instruction vmcall needs to run on ring0. The root user is in ring3.

    Wen Congyang

    > --
    > Gleb.

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