Re: Driver domain reboot, xenstore backend branches

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 15/12/15 16:38, Pavlo Suikov wrote:
> Hi,
>
> got a question regarding reboot of a driver domain in Xen 4.5. What we
> currently have is Xen 4.5, Linux in Dom0 and DomD and Android in DomU.
>
> On DomU start, xenstore is populated with both backend and frontend
> branches for all the devices present in config. So it looks like this
> (DomD is domain 1, DomU is domain 2):
>
> ...
> /local/domain/1/backend/vif
> ...
> /local/domain/2/device/vif
> ...
>
> After issuing DomD reboot, backend branches which correspond to it are
> cleared, so xenstore looks like this:
>
> ...
> /local/domain/2/device/vif
> ...
>
> Still have frontend branch, but no corresponding backend node, since
> DomU was not restarted and DomD config knows nothing about these devices.
>
> We want to have DomD reboot with split driver restart and backend
> reconnect to hanging frontends. It there a right way to do it in Xen
> 4.5 (or 4.6 for that matter if something new was introduced in this
> version exactly)?
>
> Devd seems to be quite a tool for the job, but it observes backend
> branches rather that filling them. Am I missing something here? How
> split drivers are supposed to work in a DomD reboot scenario?

They don't.

XenServer observed the same kinds of issues when experimenting with
device driver domains.

We even investigated several options such as rebooting the driver domain
with the same domid, and suspending/resuming the affected guests to
cause them to reconnect their PV rings.  Neither of these approaches
worked cleanly.

We came to the conclusion that rebootable driver domains is an area
which will need active development work to function.

~Andrew

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