[Xen-tools] irc exchange on hot-removal of devices.

[harry <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>]

harry_ dansmith: do you happen to know how hot-removal of a device from
a domain is supposed to workusing xenbus + xenstore?
murb I know it is a good way of causing a domain to go boom with xen2
harry_ dansmith: do the tools delete the FE and BE xenstore directories?
If so, how does sequencing the teardown of the shared page work?
dansmith harry_: well,
dansmith harry_: I know that hot-adding block devices works
dansmith harry_: and if/when my patch gets applied, it'll not crash the
tools as well :)
harry_ dansmith: the hot add case is simpler
dansmith harry_: I'm not quite sure about the remove case, no
harry_ dansmith: have you got any idea what the tools are supposed to do
to xenstore to implement a remove correctly?
dansmith harry_: I don't think the tools clean up the store directories,
no.. 
dansmith harry_: sequencing you mean?  No, I've been a little
disconnected from the store work recently, so I'm not sure what the
procedure is
harry_ dansmith: do you know what change is made in the store to trigger
the removal of a device from a domain?
dansmith harry_: I don't think the tools currently do anything about a
remove.  If I remember correctly, the block-remove still tries (and
fails) to send a control message to do the job, so you may be breaking
new ground :)
dansmith harry_: I guess you need a flag somewhere that tells the front
and backends to start cleaning up and stop any further traffic, right?
harry_ dansmith: Yes, I was assuming that something like that was going
to be required.
harry_ dansmith:  I just didn't want to invent a mechanism if there was
already one designed or in plan.
harry_ dansmith: who would know what the intention is here?
dansmith harry_: well, I'm obviously not the authority :) have you
looked that the blkif front/backend drivers to see what triggers them?
dansmith harry_: probably rusty or christian
harry_ dansmith: the remove entrypoint is definitely called when you
unload a module.  I haven't managed to work out if it gets called when
the tools delete a device from the store and if so whether there's any
way of getting the teardown sequencing right.
dansmith harry_: let me look at the tools code real quick...
harry_ I sent rusty a note, copying christian and xen-tools.  No reply
so far.
dansmith harry_: yea, I can tell you for sure that xend does nothing
store-related on a block-remove.  Have you looked at the store-ized
vmtools?  I bet rusty would've implemented it there while developing
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harry_ Last-time I looked at vm-tools they were lagging the xend code.
I was under the impression they were on hold until xend was working.
dansmith lemme check
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dansmith harry_: yep, you're right, no block-teardown-via-the-store
support in there :)
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harry_ dansmith:  There needs to be a writeup of both creation and
correct teardown of a BE<->FE connection and example code for people to
follow before xenbus is generally usable.  Unless we want to end up with
a different implementation for every driver :-)
dansmith harry_: I agree 100%
dansmith harry_: if I knew the procedure, I'd do it right away :)
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harry_ dansmith: I will copy this exchange to Christian and Rusty.  For
the time being, I can leave the removal cases unimplemented.


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