[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] Driver domains and hotplug scripts, redux

2012/1/11 Ian Campbell <Ian.Campbell@xxxxxxxxxx>:
> On Wed, 2012-01-11 at 11:50 +0000, Roger Pau Monnà wrote:
>> Hello,
>> This comes from my experience with Xen and hotplug scripts, and it
>> might be wrong, since I wasn't able to find any document explaining
>> exactly how hotplug execution works and who does what. I'm gonna try
>> to list the sequence of events that happens when a device is added (I
>> really don't want to keep on with the discusion if this is a protocol
>> or not):
>> 1. Toolstack writes: /local/domain/0/backend/<vbd or vif>/... with "state = 
>> 1".
>> 2. Kernel acks xenstore backend device creation, creates the device
>> and sets backend "state = 2".
>> 3. xenbackendd notices backend device with "state == 2" and launches
>> hotplug script.
> In the Linux I think state == 2 corresponds to the generation of a
> uevent which triggers udev to run the hotplug script. I'm not 100% sure
> that all devices do this at the same point though.
>> 4. Hotplug script executes necessary actions and sets backend
>> "hotplug-status = connected".
>> 5. Kernel notices "hotplug-status == connected", plugs the device, and
>> sets xenstore backend device "state = 4".
> I think 4+5 are correct for Linux netback but for blkback it actually
> waits for phys-dev (or whatever it's real name is) to be written wather
> than the hotplug-status node.
>> This is true on NetBSD, because there aren't any userspace hotplug
>> devices, someone should probably add the missing bits if the device is
>> implemented in userspace (I'm not really sure of what happens inside
>> the kernel in #2 and #5, specially when using blktap or qdisk).
> Nothing happens in the kernel for qdisk. It is a separate backend path
> which the kernel doesn't watch or have a driver for.
> blktap1 behaves a lot like blkback, I think.
> blktap2 doesn't use xenbus IIRC, rather it is created via userspace
> tools/libraries. There _might_ be some hotplug script interaction which
> causes the phys-dev node to get written to the associated blkback device
> but I think this is not the case and the toolstack just writes the
> phys-dev because it knows what it is from when it created it.
>> Regarding device shutdown/destroy:
> We need to consider 3 cases:
> Â Â Â* guest initiated graceful shutdown
> Â Â Â* toolstack initiated graceful shutdown
> Â Â Â* toolstack initiated forceful destroy.
>> 1. Guest sets frontend state to 6 (closed)
>> 2. Kernel unplugs the device and sets backend "state = 6".
>> 3. xenbackendd notices device with "state == 6", and performs the
>> necessary cleanup.
>> 3. Toolstack notices device with "state == 6" and removes xenstore
>> backend entries.
> At least some backend/frontends make use of state 5 as part of this,
> probably at #1 or #2.
> The ordering of #1 and #2 probably depends on whether the frontend or
> the backend initiates things.

I don't think graceful shutdown presents any problems, since it can be
handled by the driver domain the same way it is handled by the
toolstack right now, and the toolstack should just wait for:

/hotplug/domain/<driverdom_id>/<vbd or vif>/<domu_id>/<device_id>/state

to be set to "closed".

The forceful shutdown however, presents some problems, and I don't
think the toolstack should remove the backend, instead we should add
something like:

/hotplug/domain/<driverdom_id>/<vbd or vif>/<domu_id>/<device_id>/destroy

And let the toolstack set this to 1 and wait for hotplug state to be
set to "closed" (just like a normal shutdown), then the driver domain
should take care of destroying the device and removing the backend and
frontend nodes, since it was the one that created them.

> The forceful destroy case is different, it is effectively:
> 1. rm backend dir in xenstore.
> Somewhere in both of these a Linux backend will generate a hotplug event
> which will cause a script to run, although in some cases the script
> can't do much because the backend dir is already gone...
>> Notice that I've used two #3, that's where the race condition happens,
>> because there's no synchronization between toolstack and
>> hotplug/xenbackendd to know when hotplug scripts have been executed
>> (however we should be able to synchronize this watching
>> "hotplug-status" instead of "state", and waiting for it to change to
>> "disconnected").
>> Now, we have to decide how to fix the shutdown/destroy race and how to
>> implement this outside of the Dom0. I'm not really sure if it's a good
>> idea to try so hard to keep this flow intact, I think it's best to try
>> to define a flow that solves our current problems, regardless of how
>> things are now, and then try to map both flows to see what should be
>> changed and how.
>> Since the device will be plugged from a Domain different than Dom0,
>> the toolstack doesn't really (and probably shouldn't) know anything
>> about which backend type will be used (phy, blktap, qdisk...). Having
>> that in mind, I don't know how can we write
>> /local/domain/<driverdom_id>/backend/... from Dom0, instead we should
>> create something like:
>> /hotplug/domain/<driverdom_id>/<vbd or vif>/<domu_id>/<device_id>/params
>> /hotplug/domain/<driverdom_id>/<vbd or vif>/<domu_id>/<device_id>/script
>> /hotplug/domain/<driverdom_id>/<vbd or vif>/<domu_id>/<device_id>/state
>> [This seem like the minimum necessary parameters, but probably there
>> are others, so add what you feel necessary]
>> With that the driver domain should be able to create
>> /local/domain/<driverdomain_id>/backend/... and the frontend also.
>> I'm not sure if we should control the execution of hotplug scripts
>> from Dom0, or instead let the driver domain decide when it's best to
>> execute each script. This adds /hotplug to xenstore, but the
>> plug/unplug sequence could be the same as the one we currently have,
>> the only change is that each driver domain is in charge of writing
>> it's own xenstore backend/frontend entries to trigger the plug
>> sequence.
>> Hope that helps, Roger.
>> (xen-devel mailing list was removed at some point during the
>> conversation, so I'm adding it again)
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.