Re: [Xen-devel] [PATCH v4 7/7] xl: allow domid to be preserved on save/restore or migrate

From: Wei Liu <wl@xen.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Anthony Perard <anthony.perard@citrix.com>,
	Ian Jackson <ian.jackson@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Wei Liu <wl@xen.org>, "Durrant, Paul" <pdurrant@amazon.co.uk>
Subject: Re: [Xen-devel] [PATCH v4 7/7] xl: allow domid to be preserved on save/restore or migrate
Date: Fri, 31 Jan 2020 16:07:48 +0000	[thread overview]
Message-ID: <20200131160748.anvywpswgmps36aq@debian> (raw)
In-Reply-To: <38d14767-eca0-6343-799c-49167271c9e7@citrix.com>

On Thu, Jan 30, 2020 at 06:20:08PM +0000, Andrew Cooper wrote:
> On 30/01/2020 17:42, Durrant, Paul wrote:
> >> -----Original Message-----
> >> From: Ian Jackson <ian.jackson@citrix.com>
> >> Sent: 30 January 2020 17:29
> >> To: Durrant, Paul <pdurrant@amazon.co.uk>
> >> Cc: xen-devel@lists.xenproject.org; Wei Liu <wl@xen.org>; Anthony Perard
> >> <anthony.perard@citrix.com>
> >> Subject: Re: [PATCH v4 7/7] xl: allow domid to be preserved on
> >> save/restore or migrate
> >>
> >> Paul Durrant writes ("[PATCH v4 7/7] xl: allow domid to be preserved on
> >> save/restore or migrate"):
> >>> This patch adds a '-D' command line option to save and migrate to allow
> >>> the domain id to be incorporated into the saved domain configuration and
> >>> hence be preserved.
> >> Thanks.
> >>
> >> In response to v3 6/6 I wrote:
> >>
> >>   I wonder if this should be done more in libxl.  Should this be a
> >>   domain property ?  Wei, Anthony ?
> >>
> >> This question remains unanswered.  I'm sorry that neither Wei nor
> >> Anthony had a chance to answer yet...
> >>
> >> Paul, do you have a reason for doing it this way ?  My inclination is
> >> that think doing it at the libxl layer would make more sense.  Do you
> >> think that would be possible ?
> >>
> > I'm not sure how it would work at the libxl level as the domid is
> > part of create_info and that it transferred by xl on migration. IIUC
> > we'd need a new libxl save record to transfer it at that level, and
> > then I'm not sure whether we'd hit an ordering problem as we'd have
> > to dig that save record out before we could actually create the
> > domain.
> 
> There is definitely an ordering problem.
> 
> I agree that it should logically be part of the libxl level of the
> stream, but none of that is even parsed until after the domain has been
> created on the destination side.
> 
> I have no idea how easy/difficult it would be to rearrange libxl to
> start parsing the migration stream before creating the domain.  I think
> there will be a lot of code relying on the domid already being valid.

This.

The other way I can think of is to specify something (domid policy??) in
create_info and apply it during domain creation. But that reeks like a
layering violation to me.

Wei.

> 
> ~Andrew

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