Re: Xen 4.18 release: Reminder about code freeze

[Juergen Gross <jgross@xxxxxxxx>]

On 13.10.23 17:16, George Dunlap wrote:
> On Fri, Oct 13, 2023 at 4:04 PM Juergen Gross <jgross@xxxxxxxx> wrote:
> > > > > Technically, dom0 has exactly the same problem as dom0less domains it boots
> > > > > before Xenstored is running and therefore it may need to know when it is
> > > > > ready to receive commands.
> > > > Umm, no, not really.
> > > >
> > > > The main difference between dom0 and a dom0less domU is, that xenstored
> > > > introduces dom0 by itself via a call of dom0_init(), while the dom0less
> > > > domUs get introduced by Xen tools in case a dom0 is coming up later. And
> > > > that XS_INTRODUCE will clobber any ring page contents, while a call of
> > > > dom0_init() won't do that.
> > > >
> > > > Dom0 (especially the kernel) is fine to start filling the ring page with
> > > > requests even before xenstored is running. It just shouldn't expect to
> > > > receive any responses right away.
> > > I am not sure what you mean by fine. You will see hang notifications if
> > > Xenstored is not started in time. Isn't why we decided to go with a different
> > > way for dom0less?
> > The main difference is that dom0 tells xenstored the connection parameters for
> > itself, so dom0 _knows_ that the ring page is setup correctly when xenstored
> > starts looking at it (it is dom0 which needs to do the ring page init).
> >
> > A dom0less domU doesn't have that negotiation with xenstored, as xenstored just
> > uses the pre-defined grant for looking at the ring page. For the domU there is
> > no way to tell that xenstored has initialized the ring page (it is not the domU
> > to do the initialization, as the XS_INTRODUCE might be sent before the domU
> > even starts running), other than the "connected" indicator in the page itself.
> Any thoughts on my patch?  From your description, it sounds like maybe
> it should be in the right direction, although it's currently missing
> memory barriers.
Yes, I think the patch is basically okay (apart from the barriers).


Juergen

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