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



On 29/04/2020 13:29, Jürgen Groß wrote:
> On 29.04.20 13:04, Igor Druzhinin wrote:
>> 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.
> 
> Wei, Ian, can you please tell me where I'm wrong?
> 
> A soft reset should restart the domain in a clean state. AFAIK libxl is
> handling that by doing kind of in-place save-restore, including calling
> xs_release_domain() and later xs_introduce_domain(). This should result
> in xenstored throwing away all state it has regarding the domain and
> then restarting with a new (internal) domain instance.
> 
> Is XAPI doing soft reset differently? Why should there be a need for
> keeping xenstored data across a soft reset?

Yes, XAPI is doing soft reset differently from libxl. You need to keep xenstore
data to at least keep backends working without the need to reinitialize them 
before entering kdump kernel. We do the same thing while entering crash kernel
in Windows after BSOD (CC Paul as he recommended this approach).
There are other reasons to not reset xenstore data.

I considered XS_INTRODUCE functionality to be part of xenstored ABI and we built
a lot of infrastructure on top of that. So I don't think it could be easily 
removed now.

Igor



 


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