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

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 29/04/2020 14:22, Paul Durrant wrote:
>> -----Original Message-----
>> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>> Sent: 29 April 2020 14:17
>> To: paul@xxxxxxx; 'Jürgen Groß' <jgross@xxxxxxxx>; 'Julien Grall' 
>> <julien@xxxxxxx>; 'Julien Grall'
>> <julien.grall.oss@xxxxxxxxx>
>> Cc: 'xen-devel' <xen-devel@xxxxxxxxxxxxxxxxxxxx>; 'Ian Jackson' 
>> <ian.jackson@xxxxxxxxxxxxx>; 'Wei Liu'
>> <wl@xxxxxxx>; andrew.cooper3@xxxxxxxxxx
>> Subject: Re: [PATCH] tools/xenstore: don't store domU's mfn of ring page in 
>> xensotred
>>
>> On 29/04/2020 13:56, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
>>>> Sent: 29 April 2020 13:50
>>>> To: Jürgen Groß <jgross@xxxxxxxx>; Julien Grall <julien@xxxxxxx>; Julien 
>>>> Grall
>>>> <julien.grall.oss@xxxxxxxxx>
>>>> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Ian Jackson 
>>>> <ian.jackson@xxxxxxxxxxxxx>; Wei Liu
>>>> <wl@xxxxxxx>; andrew.cooper3@xxxxxxxxxx; Paul Durrant <paul@xxxxxxx>
>>>> Subject: Re: [PATCH] tools/xenstore: don't store domU's mfn of ring page 
>>>> in xensotred
>>>>
>>>> On 29/04/2020 13:29, Jürgen Groß wrote:
>>>>>
>>>>> 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).
>>>
>>> IIRC I recommended not involving Xen or the toolstack in entering the crash 
>>> kernel... they don't
>> need to know. Windows quite happily enters its crash kernel, rebuilds its 
>> Xen interfaces and re-
>> attaches to PV backends without any external intervention.
>>
>> In case of kdump toolstack certainly needs to know as it gets shutdown code 
>> 5 and
>> needs to restart the domain.
>>
> 
> The toolstack needs to restart the domain once the crash has completed, yes.

To clarify, what I meant is that once crash happened (before kdump kernel is 
loaded)
toolstack gets code 5 and then it's supposed to call soft reset and unpause the 
domain
to actually load into crash kernel.

I didn't mean that after crash kernel is finished the domain has to be 
restarted - that's
obvious.

Igor