Re: [PATCH] tools/xenstore: don't store domU's mfn of ring page in xensotred

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 29/04/2020 10:22, Julien Grall wrote:
> Hi Juergen,
> 
> On 29/04/2020 06:51, Jürgen Groß wrote:
>>
>> Recreating the event channel would be fine, but I don't see why it
>> would ever be needed. And XS_INTRODUCE is called only at domain creation
>> time today, so there is obviously no need for repeating this call.
>>
>> Maybe the idea was to do this after sending a XS_RESUME command, which
>> isn't used anywhere in Xen and is another part of Xenstore which doesn't
>> make any sense today.
> 
> Commit f6cc37ea8ac71385b60507c034519f304da75f4c "tools/oxenstored: port 
> XS_INTRODUCE evtchn rebind function from cxenstored" added the exact same 
> behavior in the OCaml XenStored last year.
> 
> This was introduced 12 years after C XenStored, so surely someone think this 
> is useful. We should check why this was introduced in OCaml XenStored (I have 
> CCed the author of the patch).
> 
> If we still think this is not useful, then you should add an explanation in 
> the commit message why the two implementations diverge and possibly update 
> the spec.

Thanks for CC, Julien.

We indeed already use this functionality in our toolstack for guest kdump
functions. It's not possible in XAPI to adopt libxl model where almost 
everything
is restarted for a domain entering kdump, so we have to use this message to
rebind xenstore evtchn after soft reset without shutting down backends and
recreating the whole subtree (frontends reconnect fine after that).

We obviously only require it for now to be present in oxenstored only.
Please don't remove this functionality if possible.

Igor