[Xen-devel] [PATCH 2/2] xen: Fix determining when domain creation is complete

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

d->creation_finished is used in several places alter behaviour depending on
whether the domain is being created, or is already running.

However, there is a latent bug if a toolstack component makes a pair of
pause/unpause calls, where creation will be considered finished prematurely.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Paul Durrant <paul.durrant@xxxxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

This is probably a backport candidate, as creation_finished was introduced in
4.8
---
 xen/common/domain.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 3abaca9..05130e2 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1004,13 +1004,6 @@ int domain_unpause_by_systemcontroller(struct domain *d)
 {
     int old, new, prev = d->controller_pause_count;
 
-    /*
-     * We record this information here for populate_physmap to figure out
-     * that the domain has finished being created. In fact, we're only
-     * allowed to set the MEMF_no_tlbflush flag during VM creation.
-     */
-    d->creation_finished = true;
-
     do
     {
         old = prev;
@@ -1022,6 +1015,20 @@ int domain_unpause_by_systemcontroller(struct domain *d)
         prev = cmpxchg(&d->controller_pause_count, old, new);
     } while ( prev != old );
 
+    /*
+     * d->controller_pause_count is initialised to 1, and the toolstack is
+     * responsible for making one unpause hypercall when it wishes the guest
+     * to start running.
+     *
+     * All other toolstack operations should make a pair of pause/unpause
+     * calls and rely on the reference counting here.
+     *
+     * Creation is considered finished when the controller reference count
+     * first drops to 0.
+     */
+    if ( new == 0 )
+        d->creation_finished = true;
+
     domain_unpause(d);
 
     return 0;
-- 
2.1.4


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