[Xen-tools] Question about xenbus

[harry <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Hi Rusty,

    I'm still trying to understand how to use xenbus.

    I think I vaguely get the device creation scenario:

    Tools create a front-end directory in the store and a back-end
directory in the store putting configuration parameters in the
directories.  The drivers watch each other's directories to communicate
the information about the shared page.

    I'm trying to consider other scenarios involving the following
events:

Tools add device to domain
Tools remove device from domain
BE domain boots
BE domain suspends
BE domain resumes
BE domain shuts down
BE domain boots
BE domain suspends
BE domain resumes
BE domain shuts down
BE driver module is installed
BE driver module is removed
FE driver module is installed
FE driver module is removed

    The scenarios involving tools adding a device to a domain seem
relatively straightforward, even when adding devices to running domains.

    Equally, installing driver modules seems plausible, even when
devices are already created in xenstore.

    Removing driver modules whilst there are created devices is more
difficult because of the problem of sequencing the teardown of the
shared page such that the FE doesn't free it before the BE has finished
using it.  But, in one of your previous notes you proposed that the FE
and BE explicitly sequence the teardown by watching for when specific
nodes are deleted from the FE and BE directories in the store and this
seems like it might work.

    I'm getting stuck on the scenarios involving tools removing a device
whilst FE and/or BE domains are running with drivers installed.

    If I understand correctly, the tools delete the FE and BE
directories in the store which leaves the FE and BE drivers without a
mechanism to sequence the correct teardown of any active communication
channels.

    Is hot-removal of devices supposed to be supported?  If so, what's
the intended mechanism for this?

Harry.


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