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    SubjectRe: [Qemu-devel] [PATCH 0/2 v3] kvm: notify host when guest panicked
    On Wed, Mar 21, 2012 at 02:19:34PM -0500, Anthony Liguori wrote:
    > On 03/21/2012 11:25 AM, Avi Kivity wrote:
    > >On 03/21/2012 06:18 PM, Corey Minyard wrote:
    > >>
    > >>>Look at drivers/char/ipmi/ipmi_msghandler.c. It has code to send panic
    > >>>event over IMPI. The code is pretty complex. Of course if we a going to
    > >>>implement something more complex than simple hypercall for panic
    > >>>notification we better do something more interesting with it than just
    > >>>saying "panic happened", like sending stack traces on all cpus for
    > >>>instance.
    > >>
    > >>I doubt that's the best example, unfortunately. The IPMI event log
    > >>has limited space and it has to be send a little piece at a time since
    > >>each log entry is 14 bytes. It just prints the panic string, nothing
    > >>else. Not that it isn't useful, it has saved my butt before.
    > >>
    > >>You have lots of interesting options with paravirtualization. You
    > >>could, for instance, create a console driver that delivered all
    > >>console output efficiently through a hypercall. That would be really
    > >>easy. Or, as you mention, a custom way to deliver panic information.
    > >>Collecting information like stack traces would be harder to
    > >>accomplish, as I don't think there is currently a way to get it except
    > >>by sending it to printk.
    > >
    > >That already exists; virtio-console (or serial console emulation) can do
    > >the job.
    > I think the use case here is pretty straight forward: if the guest
    > finds itself in bad place, it wants to indicate that to the host.
    > We shouldn't rely on any device drivers or complex code. It should
    > be as close to a single instruction as possible that can run even if
    > interrupts are disabled.
    > An out instruction fits this very well. I think a simple protocol like:
    > inl PORT -> returns a magic number indicating the presence of qemucalls
    > inl PORT+1 -> returns a bitmap of supported features
    Sigh, one more PV isa device.

    > outl PORT+1 -> data reg1
    > outl PORT+2 -> data reg2
    > outl PORT+N -> data regN
    > outl PORT -> qemucall of index value with arguments 1..N
    And you think you can trust panicked SMP guest to not call this on
    multiple cpus simultaneously?

    > Regards,
    > Anthony Liguori
    > >
    > >In fact the feature can be implemented 100% host side by searching for a
    > >panic string signature in the console logs.
    > >


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