[xen stable-4.17] cxenstored: wait until after reset to notify dom0less domains

[patchbot@xxxxxxx]

commit 65e1f3d289d7b35ee8eceaca1d5c4346d5562e04
Author:     George Dunlap <george.dunlap@xxxxxxxxx>
AuthorDate: Tue Nov 14 13:52:27 2023 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Nov 14 13:52:27 2023 +0100

    cxenstored: wait until after reset to notify dom0less domains
    
    Commit fc2b57c9a ("xenstored: send an evtchn notification on
    introduce_domain") introduced the sending of an event channel to the
    guest when first introduced, so that dom0less domains waiting for the
    connection would know that xenstore was ready to use.
    
    Unfortunately, it was introduced in introduce_domain(), which 1) is
    called by other functions, where such functionality is unneeded, and
    2) after the main XS_INTRODUCE call, calls domain_conn_reset().  This
    introduces a race condition, whereby if xenstored is delayed, a domain
    can wake up, send messages to the buffer, only to have them deleted by
    xenstore before finishing its processing of the XS_INTRODUCE message.
    
    Move the connect-and-notfy call into do_introduce() instead, after the
    domain_conn_rest(); predicated on the state being in the
    XENSTORE_RECONNECT state.
    
    (We don't need to check for "restoring", since that value is always
    passed as "false" from do_domain_introduce()).
    
    Also take the opportunity to add a missing wmb barrier after resetting
    the indexes of the ring in domain_conn_reset.
    
    This change will also remove an extra event channel notification for
    dom0 (because the notification is now done by do_introduce which is not
    called for dom0.) The extra dom0 event channel notification was only
    introduced by fc2b57c9a and was never present before. It is not needed
    because dom0 is the one to tell xenstored the connection parameters, so
    dom0 has to know that the ring page is setup correctly by the time
    xenstored starts looking at it. It is dom0 that performs the ring page
    init.
    
    Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxx>
    Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    master commit: e58bd71c097d04f001e0c6c2868a0aec03d25c63
    master date: 2023-10-17 13:56:55 -0700
---
 tools/xenstore/xenstored_domain.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/tools/xenstore/xenstored_domain.c 
b/tools/xenstore/xenstored_domain.c
index 6074df210c..ec91334c83 100644
--- a/tools/xenstore/xenstored_domain.c
+++ b/tools/xenstore/xenstored_domain.c
@@ -558,6 +558,7 @@ static void domain_conn_reset(struct domain *domain)
 
        domain->interface->req_cons = domain->interface->req_prod = 0;
        domain->interface->rsp_cons = domain->interface->rsp_prod = 0;
+       xen_wmb();
 }
 
 /*
@@ -623,12 +624,6 @@ static struct domain *introduce_domain(const void *ctx,
                /* Now domain belongs to its connection. */
                talloc_steal(domain->conn, domain);
 
-               if (!restore) {
-                       /* Notify the domain that xenstore is available */
-                       interface->connection = XENSTORE_CONNECTED;
-                       xenevtchn_notify(xce_handle, domain->port);
-               }
-
                if (!is_master_domain && !restore)
                        fire_watches(NULL, ctx, "@introduceDomain", NULL,
                                     true, NULL);
@@ -669,6 +664,13 @@ int do_introduce(const void *ctx, struct connection *conn,
 
        domain_conn_reset(domain);
 
+       if (domain->interface != NULL &&
+           domain->interface->connection == XENSTORE_RECONNECT) {
+               /* Notify the domain that xenstore is available */
+               domain->interface->connection = XENSTORE_CONNECTED;
+               xenevtchn_notify(xce_handle, domain->port);
+       }
+
        send_ack(conn, XS_INTRODUCE);
 
        return 0;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.17