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

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 29/04/2020 11:49, Jürgen Groß wrote:
> On 29.04.20 12:39, Igor Druzhinin wrote:
>> 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.
> 
> If I read handling in libxl correctly, in the soft reset case XS_RELEASE
> is issued before doing another XS_INTRODUCE. XS_RELEASE will result in
> xenstored throwing away its related struct domain, so XS_INTRODUCE will
> be possible again.

>From what I remember it was not possible to keep xenstored data for a domain
and at the same time perform release-introduce cycle (at least in oxenstored).
It also involved firing @releaseDomain which caused havoc in other part of
the toolstack.

Igor