Re: Xen 4.18 release: Reminder about code freeze

[Juergen Gross <jgross@xxxxxxxx>]

On 13.10.23 13:22, Julien Grall wrote:
> Hi George,
>
> On 13/10/2023 11:16, George Dunlap wrote:
> > On Thu, Oct 12, 2023 at 11:36 PM Stefano Stabellini
> > <sstabellini@xxxxxxxxxx> wrote:
> > > On Thu, 12 Oct 2023, George Dunlap wrote:
> > > > > > Stop tinkering in the hope that it hides the problem.  You're only
> > > > > > making it harder to fix properly.
> > > > > Making it harder to fix properly would be a valid reason not to commit
> > > > > the (maybe partial) fix. But looking at the fix again:
> > > > >
> > > > > diff --git a/tools/xenstored/domain.c b/tools/xenstored/domain.c
> > > > > index a6cd199fdc..9cd6678015 100644
> > > > > --- a/tools/xenstored/domain.c
> > > > > +++ b/tools/xenstored/domain.c
> > > > > @@ -989,6 +989,7 @@ static struct domain *introduce_domain(const void *ctx,
> > > > >                  talloc_steal(domain->conn, domain);
> > > > >
> > > > >                  if (!restore) {
> > > > > +                       domain_conn_reset(domain);
> > > > >                          /* Notify the domain that xenstore is available */
> > > > >                          interface->connection = XENSTORE_CONNECTED;
> > > > >                          xenevtchn_notify(xce_handle, domain->port);
> > > > > @@ -1031,8 +1032,6 @@ int do_introduce(const void *ctx, struct connection 
> > > > > *conn,
> > > > >          if (!domain)
> > > > >                  return errno;
> > > > >
> > > > > -       domain_conn_reset(domain);
> > > > > -
> > > > >          send_ack(conn, XS_INTRODUCE);
> > > > >
> > > > > It is a 1-line movement. Textually small. Easy to understand and to
> > > > > revert. It doesn't seem to be making things harder to fix? We could
> > > > > revert it any time if a better fix is offered.
> > > > >
> > > > > Maybe we could have a XXX note in the commit message or in-code
> > > > > comment?
> > > > It moves a line from one function (do_domain_introduce()) into a
> > > > completely different function (introduce_domain()), nested inside two
> > > > if() statements; with no analysis on how the change will impact
> > > > things.
> > > I am not the original author of the patch, and I am not the maintainer
> > > of the code, so I don't feel I have the qualifications to give you the
> > > answers you are seeking. Julien as author of the patch and xenstore
> > > reviewer might be in a better position to answer. Or Juergen as xenstore
> > > maintainer.
> > I understand that; my main point is that the change is more complex
> > than you're characterizing it.  This is information necessary to
> > understand whether the patch is correct, but it's not in the patch
> > description, nor in the subsequent thread back in May.
> >
> > > > Are there any paths through do_domain_introduce() that now *won't* get
> > > > a domain_conn_reset() call?  Is that OK?
> > > Yes, the already-introduced and the restore code paths. The operations in
> > > the already-introduced or the restore code paths seem simple enough not
> > > to require a domain_conn_reset. Julien and Juergen should confirm.
> > There is no "restore" codepath through do_domain_introduce(); it
> > passes "false" for the "restore" argument.  So we  only have two paths
> > to consider through do_domain_introduce(): The "not introduced and not
> > restoring" path, and the "already-introduced" path.
> >
> > I'm not sure what the "simple" elements on the branch in
> > introduce_domain() have to do with whether the content of the page
> > needs to be cleaned up.  As I said, I don't 100% understand this code,
> > but it seems like if anything, the reset would be *more* important to
> > have in the "reintroduce" case than in the "initial introduction"
> > case, since I'd expect the "initial introduction" case to be empty
> > already.
> Indeed, there should be no watches/transactions/buffered I/O for the initial 
> introduction. However, the function is also clear part of the interface because 
> we can't guaranteed it was zeroed.
> The latter matter for the initial introduction. I believe the rest is just 
> called for simplicity.
> > > Doesn't it seem weird to you that we set a connection to CONNECTED,
> > > notify the domain that it is ready to go, and only *after* that we reset
> > > the connection to zero?
> > >
> > > What happens if a domain starts using the connection as soon as it
> > > receives the event channel notification and before domain_conn_reset is
> > > called?
> > Yes, it does seem weird, which is why I said the following. :-)
> >
> > > > I mean, it certainly seems strange to set the state to CONNECTED, send
> > > > off an event channel, and then after that delete all watches /
> > > > transactions / buffered data and so on;
> > But just because the current code is probably wrong, doesn't mean that
> > the modified code is probably correct.
> >
> > If the problem is the delay between the xenevtchn_notify() in
> > introduce_domain() and the domain_conn_reset() afterwards in
> > do_domain(), would it make sense instead to move the notification into
> > do_introduce(), after the domain_conn_reset()?  It is, after all, in
> > response to XS_INTRODUCE that we want to send the notification, not in
> > dom0_init() or read_state_connection() (which seems to be more about
> > restoring a domain).
> I understand that the event channel notification was specifically added for 
> dom0less. But I don't see why we don't want to send it to dom0 as well.
> 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.


Juergen

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