[PATCH v2 5/6] tools/xenstored: partially handle domains without a shared ring

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Failure to map the shared ring and thus establish a xenstore
connection with a domain shouldn't prevent the "@introduceDomain"
watch from firing, likewise with "@releaseDomain".

In order to handle such events properly xenstored should keep track of
the domains even if the shared communication ring cannot be mapped.
This was partially the case due to the restore mode, which needs to
handle domains that have been destroyed between the save and restore
period. This patch extends on the previous limited support of
temporary adding a domain without a valid interface ring, and modifies
check_domains to keep domains without an interface on the list.

Handling domains without a valid shared ring is required in order to
support domain without a grant table, since the lack of grant table
will prevent the mapping of the xenstore ring grant reference.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
oxenstored will need a similar treatment once grant mapping is used
there. For the time being it still works correctly because it uses
foreign memory to map the shared ring, and that will work in the
absence of grant tables on the domain.
---
Changes since v1:
 - New in this version.
---
 tools/xenstore/xenstored_domain.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/tools/xenstore/xenstored_domain.c 
b/tools/xenstore/xenstored_domain.c
index 9fb78d5772..150c6f082e 100644
--- a/tools/xenstore/xenstored_domain.c
+++ b/tools/xenstore/xenstored_domain.c
@@ -119,6 +119,11 @@ static int writechn(struct connection *conn,
        struct xenstore_domain_interface *intf = conn->domain->interface;
        XENSTORE_RING_IDX cons, prod;
 
+       if (!intf) {
+               errno = ENODEV;
+               return -1;
+       }
+
        /* Must read indexes once, and before anything else, and verified. */
        cons = intf->rsp_cons;
        prod = intf->rsp_prod;
@@ -149,6 +154,11 @@ static int readchn(struct connection *conn, void *data, 
unsigned int len)
        struct xenstore_domain_interface *intf = conn->domain->interface;
        XENSTORE_RING_IDX cons, prod;
 
+       if (!intf) {
+               errno = ENODEV;
+               return -1;
+       }
+
        /* Must read indexes once, and before anything else, and verified. */
        cons = intf->req_cons;
        prod = intf->req_prod;
@@ -176,6 +186,9 @@ static bool domain_can_write(struct connection *conn)
 {
        struct xenstore_domain_interface *intf = conn->domain->interface;
 
+       if (!intf)
+               return false;
+
        return ((intf->rsp_prod - intf->rsp_cons) != XENSTORE_RING_SIZE);
 }
 
@@ -183,7 +196,8 @@ static bool domain_can_read(struct connection *conn)
 {
        struct xenstore_domain_interface *intf = conn->domain->interface;
 
-       if (domain_is_unprivileged(conn) && conn->domain->wrl_credit < 0)
+       if ((domain_is_unprivileged(conn) && conn->domain->wrl_credit < 0) ||
+           !intf)
                return false;
 
        return (intf->req_cons != intf->req_prod);
@@ -268,14 +282,7 @@ void check_domains(void)
                                domain->shutdown = true;
                                notify = 1;
                        }
-                       /*
-                        * On Restore, we may have been unable to remap the
-                        * interface and the port. As we don't know whether
-                        * this was because of a dying domain, we need to
-                        * check if the interface and port are still valid.
-                        */
-                       if (!dominfo.dying && domain->port &&
-                           domain->interface)
+                       if (!dominfo.dying)
                                continue;
                }
                if (domain->conn) {
@@ -450,8 +457,6 @@ static struct domain *introduce_domain(const void *ctx,
        if (!domain->introduced) {
                interface = is_master_domain ? xenbus_map()
                                             : map_interface(domid);
-               if (!interface && !restore)
-                       return NULL;
                if (new_domain(domain, port, restore)) {
                        rc = errno;
                        if (interface) {
@@ -505,7 +510,8 @@ int do_introduce(struct connection *conn, struct 
buffered_data *in)
        if (!domain)
                return errno;
 
-       domain_conn_reset(domain);
+       if (domain->interface)
+               domain_conn_reset(domain);
 
        send_ack(conn, XS_INTRODUCE);
 
-- 
2.33.0