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

From: Igor Druzhinin <igor.druzhinin@citrix.com>
To: paul@xen.org, "'Jürgen Groß'" <jgross@suse.com>,
	"'Julien Grall'" <julien@xen.org>,
	"'Julien Grall'" <julien.grall.oss@gmail.com>
Cc: 'xen-devel' <xen-devel@lists.xenproject.org>,
	'Ian Jackson' <ian.jackson@eu.citrix.com>, 'Wei Liu' <wl@xen.org>,
	andrew.cooper3@citrix.com
Subject: Re: [PATCH] tools/xenstore: don't store domU's mfn of ring page in xensotred
Date: Wed, 29 Apr 2020 14:16:43 +0100	[thread overview]
Message-ID: <0eaea23f-8b4a-2c67-2fc4-827cf26dbd8d@citrix.com> (raw)
In-Reply-To: <000001d61e25$97f86530$c7e92f90$@xen.org>

On 29/04/2020 13:56, Paul Durrant wrote:
>> -----Original Message-----
>> From: Igor Druzhinin <igor.druzhinin@citrix.com>
>> Sent: 29 April 2020 13:50
>> To: Jürgen Groß <jgross@suse.com>; Julien Grall <julien@xen.org>; Julien Grall
>> <julien.grall.oss@gmail.com>
>> Cc: xen-devel <xen-devel@lists.xenproject.org>; Ian Jackson <ian.jackson@eu.citrix.com>; Wei Liu
>> <wl@xen.org>; andrew.cooper3@citrix.com; Paul Durrant <paul@xen.org>
>> 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.

And I'm not completely sure it's possible to enter kdump without calling soft reset
at all. Even if it's possible I'd be cautious to do it in production for the future.

Igor