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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----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.

> 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.
> 

If it is not possible at the moment then IMO it should be made so; having soft 
reset in the toolstack is a layering violation IMO.

  Paul