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Re: Xen 4.18 release: Reminder about code freeze
- To: Juergen Gross <jgross@xxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- From: Julien Grall <julien@xxxxxxx>
- Date: Fri, 13 Oct 2023 15:51:45 +0100
- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Henry Wang <Henry.Wang@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, "community.manager@xxxxxxxxxxxxxx" <community.manager@xxxxxxxxxxxxxx>
- Delivery-date: Fri, 13 Oct 2023 14:51:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Hi Juergen,
On 13/10/2023 15:47, Juergen Gross wrote:
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.
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?
Cheers,
--
Julien Grall
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