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

[Jürgen Groß <jgross@xxxxxxxx>]

On 29.04.20 12:39, Igor Druzhinin wrote:
> On 29/04/2020 10:22, Julien Grall wrote:
> > Hi Juergen,
> >
> > On 29/04/2020 06:51, Jürgen Groß wrote:
> > > Recreating the event channel would be fine, but I don't see why it
> > > would ever be needed. And XS_INTRODUCE is called only at domain creation
> > > time today, so there is obviously no need for repeating this call.
> > >
> > > Maybe the idea was to do this after sending a XS_RESUME command, which
> > > isn't used anywhere in Xen and is another part of Xenstore which doesn't
> > > make any sense today.
> > Commit f6cc37ea8ac71385b60507c034519f304da75f4c "tools/oxenstored: port XS_INTRODUCE 
> > evtchn rebind function from cxenstored" added the exact same behavior in the OCaml 
> > XenStored last year.
> >
> > This was introduced 12 years after C XenStored, so surely someone think this is 
> > useful. We should check why this was introduced in OCaml XenStored (I have CCed 
> > the author of the patch).
> >
> > If we still think this is not useful, then you should add an explanation in the 
> > commit message why the two implementations diverge and possibly update the spec.
> Thanks for CC, Julien.
>
> We indeed already use this functionality in our toolstack for guest kdump
> functions. It's not possible in XAPI to adopt libxl model where almost 
> everything
> is restarted for a domain entering kdump, so we have to use this message to
> rebind xenstore evtchn after soft reset without shutting down backends and
> recreating the whole subtree (frontends reconnect fine after that).
>
> We obviously only require it for now to be present in oxenstored only.
> Please don't remove this functionality if possible.
If I read handling in libxl correctly, in the soft reset case XS_RELEASE
is issued before doing another XS_INTRODUCE. XS_RELEASE will result in
xenstored throwing away its related struct domain, so XS_INTRODUCE will
be possible again.


Juergen