Re: [PATCH RFC 1/4] xen: evtchn: make evtchn_reset() ready for soft reset

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2015-06-05 at 09:52 +0100, Jan Beulich wrote:
> >>> On 04.06.15 at 17:47, <tim@xen.org> wrote:
> > At 16:19 +0100 on 04 Jun (1433434780), David Vrabel wrote:
> >> On 04/06/15 15:05, Tim Deegan wrote:
> >> > At 15:35 +0200 on 03 Jun (1433345719), Vitaly Kuznetsov wrote:
> >> >> We need to close all event channel so the domain performing soft reset
> >> >> will be able to open them back. Interdomain channels are, however,
> >> >> special. We need to keep track of who opened it as in (the most common)
> >> >> case it was opened by the control domain we won't be able (and allowed) to
> >> >> re-establish it.
> >> > 
> >> > 
> >> > I'm not sure I understand -- can you give an example of what this is
> >> > avoiding?  I would have thought that the kexec'ing VM needs to tear
> >> > down _all_ its connections and then restart the ones it wantrs in the
> >> > new OS.
> >> 
> >> There are some that are in state ECS_UNBOUND (console, xenstore, I
> >> think) at start of day that are allocated on behalf of the domain by the
> >> toolstack.  The kexec'd image will expect them in these same state.
> > 
> > I see.  But does that not come under "tear down all its connections
> > and then restart the ones it wants"?  I.e. should the guest not reset
> > all its channels and then allocate fresh console & xenstore ones?
> 
> But just like during boot the guest doesn't (and can't) created these,
> it also can't do so after kexec. The remote side needs to be set up
> for it, so it can simply bind to them. I don't think this can work without
> tool stack involvement.

Which IIRC is pretty much the logic which lead to the more toolstack
centric approach in the other series. Are you suggesting something in
the middle?