Re: [Xen-devel] [PATCH] xenbus: Fix loopback event channel assuming domain 0

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 10/07/2011 03:52 AM, Ian Campbell wrote:
> On Thu, 2011-10-06 at 19:32 +0100, Daniel De Graaf wrote:
>> On 10/06/2011 01:53 PM, Ian Jackson wrote:
>>> Daniel De Graaf writes ("[Xen-devel] [PATCH] xenbus: Fix loopback event 
>>> channel assuming domain 0"):
>>>> The xenbus event channel established in xenbus_init is intended to be a
>>>> loopback channel, but the remote domain was hardcoded to 0; this will
>>>> cause the channel to be unusable when xenstore is not being run in
>>>> domain 0.
>>>
>>> I'm not sure I understand this.
>>>
>>> ...
>>>>            /* Next allocate a local port which xenstored can bind to */
>>>>            alloc_unbound.dom        = DOMID_SELF;
>>>> -          alloc_unbound.remote_dom = 0;
>>>> +          alloc_unbound.remote_dom = DOMID_SELF;
>>>
>>> The comment doesn't suggest that this is supposedly a loopback channel
>>> (ie one for use by the xenbus client for signalling to itself,
>>> somehow).
>>
>> The event channel being changed here is a loopback event channel exposed in
>> /proc/xen/xsd_port, which xenstored binds to. This code is only used for the
>> initial domain; otherwise, the shared info page is used.
> 
> How does this change impact the regular dom0? It will be expecting a
> xenstored to startup locally when in reality it actually needs to wait
> for another domain and then connect to that.

This change does not attempt to address the regular dom0, except for not
breaking existing setups where xenstored resides in dom0.

> Diego Ongaro did some work several years ago on this issue, it was most
> recently re-posted by Alex Zeffert, patches against xen-unstable and the
> linux-2.6.18 tree:
> http://lists.xensource.com/archives/html/xen-devel/2009-03/msg01484.html
> http://lists.xensource.com/archives/html/xen-devel/2009-03/msg01488.html
> 
> Part of the trick was to fixup the kernel side in a way which was
> compatible with both existing Xen releases while also supporting new
> releases which support both stub and non-stub xenstore. To do this Diego
> setup a lazy xenbus initialisation with a state machine to track which
> case was active, with transitions triggered either from the local-mmap
> of /proc/xen/xenbus_xsd (so was backwards compatible) or an ioctl called
> by the tool which builds the stub domain to tell the dom0 xenbus code
> which domain/mfn/evtchn contains the xenstored and dom0's connection to
> it (the patcheset includes a cut-down builder which works without
> xenstore).
> 
> http://lists.xensource.com/archives/html/xen-devel/2009-03/msg01487.html
> is the key kernel side patch in that regard.
> 
> Diego did some initial work with xenstored in a Linux domU, but I think
> the final patchset was stub-xenstored (i.e. ocaml xenstored on minios,
> possibly C xenstored on minios too), so I'm not sure about the xenstored
> in Linux domU use case.
> 
> Some of the more trivial bits of this series were committed but the real
> meat wasn't really pushed through.
> 

Thanks for pointing out that series; I hadn't seen it yet. The setup I am
currently using has a non-Linux dom0, so the state machine in dom0 was not
required. A separate minios-based xenstored is the eventual goal; this patch
just avoids creating a broken event channel in an initial domain whose
domain ID is not 0.

I do have a more complex version of this patch that replaces the initial
domain check with a check on the start_info structure so that an initial
domain can have xenstore information placed in its start_info field like
any other domain; would this be of interest?

>>> Rather it's supposed to be a channel to xenstore.  So the remote
>>> domain should be the xenstore domain, which should come from the
>>> shared info page.
>>>
>>> Have you actually tested this with a separate xenstored domain ?
>>>
>>> Ian.
>>>
>>
>> The test setup that exposed this issue is having a non-dom0 Linux domain
>> running xenstored (in addition to other services); this domain is started
>> with the SIF_INITDOMAIN flag set. It has been tested and can start other
>> domains with references back to the xenstored running there; the local
>> kernel is able to communicate with the locally running xenstore to provide
>> backend services.
>>
>> The test for xen_initial_domain() here might better be replaced with a
>> check on xen_start_info->store_evtchn which should be a valid event channel
>> on all domains except the domain running xenstored. This seems like a more
>> elegant solution than relying on the SIF_INITDOMAIN flag to determine the
>> location of xenstore.
>>
> 

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