x86/mm: Improve ring management for memory events. Do not lose guest events

This patch is an amalgamation of the work done by Olaf Hering <olaf@aepfle.de>
and our work.

It combines logic changes to simplify the memory event API, as well as
leveraging wait queues to deal with extreme conditions in which too many
events are generated by a guest vcpu.

In order to generate a new event, a slot in the ring is claimed. If a
guest vcpu is generating the event and there is no space, it is put on a
wait queue. If a foreign vcpu is generating the event and there is no
space, the vcpu is expected to retry its operation. If an error happens
later, the function returns the claimed slot via a cancel operation.

Thus, the API has only four calls: claim slot, cancel claimed slot, put
request in the ring, consume the response.

With all these mechanisms, no guest events are lost.
Our testing includes 1. ballooning down 512 MiBs; 2. using mem access n2rwx, in
which every page access in a four-vCPU guest results in an event, with no vCPU
pausing, and the four vCPUs touching all RAM. No guest events were lost in
either case, and qemu-dm had no mapping problems.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>

1 files changed, 18 insertions, 4 deletions

diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 6441546401..dbfb8b39f3 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -13,6 +13,7 @@
 #include <xen/nodemask.h>
 #include <xen/radix-tree.h>
 #include <xen/multicall.h>
+#include <xen/wait.h>
 #include <public/xen.h>
 #include <public/domctl.h>
 #include <public/sysctl.h>
@@ -182,7 +183,9 @@ struct mem_event_domain
 {
     /* ring lock */
     spinlock_t ring_lock;
-    unsigned int req_producers;
+    /* The ring has 64 entries */
+    unsigned char foreign_producers;
+    unsigned char target_producers;
     /* shared page */
     mem_event_shared_page_t *shared_page;
     /* shared ring page */
@@ -191,6 +194,14 @@ struct mem_event_domain
     mem_event_front_ring_t front_ring;
     /* event channel port (vcpu0 only) */
     int xen_port;
+    /* mem_event bit for vcpu->pause_flags */
+    int pause_flag;
+    /* list of vcpus waiting for room in the ring */
+    struct waitqueue_head wq;
+    /* the number of vCPUs blocked */
+    unsigned int blocked;
+    /* The last vcpu woken up */
+    unsigned int last_vcpu_wake_up;
 };
 
 struct mem_event_per_domain
@@ -614,9 +625,12 @@ static inline struct domain *next_domain_in_cpupool(
  /* VCPU affinity has changed: migrating to a new CPU. */
 #define _VPF_migrating       3
 #define VPF_migrating        (1UL<<_VPF_migrating)
- /* VCPU is blocked on memory-event ring. */
-#define _VPF_mem_event       4
-#define VPF_mem_event        (1UL<<_VPF_mem_event)
+ /* VCPU is blocked due to missing mem_paging ring. */
+#define _VPF_mem_paging      4
+#define VPF_mem_paging       (1UL<<_VPF_mem_paging)
+ /* VCPU is blocked due to missing mem_access ring. */
+#define _VPF_mem_access      5
+#define VPF_mem_access       (1UL<<_VPF_mem_access)
 
 static inline int vcpu_runnable(struct vcpu *v)
 {