[PATCH v2 2/2] Xen/mem_event: Prevent underflow of vcpu pause counts

[Andrew Cooper <andrew.cooper3@citrix.com>]

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>
CC: Andres Lagar-Cavilla <andres@lagarcavilla.org>
CC: Razvan Cojocaru <rcojocaru@bitdefender.com>
CC: Aravindh Puthiyaparambil <aravindp@cisco.com>

--
v2:
 * Allow for multiple pause refcounts
---
 xen/arch/x86/hvm/hvm.c          |    2 +-
 xen/arch/x86/mm/mem_event.c     |   31 +++++++++++++++++++++++++++++++
 xen/arch/x86/mm/mem_sharing.c   |    4 ++--
 xen/arch/x86/mm/p2m.c           |    8 ++++----
 xen/include/asm-x86/mem_event.h |    3 +++
 xen/include/xen/sched.h         |    3 +++
 6 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index ef2411c..efd79b8 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -6113,7 +6113,7 @@ static int hvm_memory_event_traps(long p, uint32_t reason,
     if ( (p & HVMPME_MODE_MASK) == HVMPME_mode_sync ) 
     {
         req.flags |= MEM_EVENT_FLAG_VCPU_PAUSED;    
-        vcpu_pause_nosync(v);   
+        mem_event_vcpu_pause(v);
     }
 
     req.gfn = value;
diff --git a/xen/arch/x86/mm/mem_event.c b/xen/arch/x86/mm/mem_event.c
index 40ae841..bdde426 100644
--- a/xen/arch/x86/mm/mem_event.c
+++ b/xen/arch/x86/mm/mem_event.c
@@ -663,6 +663,37 @@ int mem_event_domctl(struct domain *d, xen_domctl_mem_event_op_t *mec,
     return rc;
 }
 
+void mem_event_vcpu_pause(struct vcpu *v)
+{
+    ASSERT(v == current);
+
+    atomic_inc(&v->mem_event_pause_count);
+    vcpu_pause_nosync(v);
+}
+
+void mem_event_vcpu_unpause(struct vcpu *v)
+{
+    int old, new, prev = v->mem_event_pause_count.counter;
+
+    /* All unpause requests as a result of toolstack responses.  Prevent
+     * underflow of the vcpu pause count. */
+    do
+    {
+        old = prev;
+        new = old - 1;
+
+        if ( new < 0 )
+        {
+            printk(XENLOG_G_WARNING
+                   "%pv mem_event: Too many unpause attempts", v);
+            return;
+        }
+
+        prev = cmpxchg(&v->mem_event_pause_count.counter, old, new);
+    } while ( prev != old );
+
+    vcpu_unpause(v);
+}
 
 /*
  * Local variables:
diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index ec99266..79188b9 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -568,7 +568,7 @@ int mem_sharing_notify_enomem(struct domain *d, unsigned long gfn,
     if ( v->domain == d )
     {
         req.flags = MEM_EVENT_FLAG_VCPU_PAUSED;
-        vcpu_pause_nosync(v);
+        mem_event_vcpu_pause(v);
     }
 
     req.p2mt = p2m_ram_shared;
@@ -609,7 +609,7 @@ int mem_sharing_sharing_resume(struct domain *d)
 
         /* Unpause domain/vcpu */
         if ( rsp.flags & MEM_EVENT_FLAG_VCPU_PAUSED )
-            vcpu_unpause(v);
+            mem_event_vcpu_unpause(v);
     }
 
     return 0;
diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index f213a39..2c7bc0f 100644
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1158,7 +1158,7 @@ void p2m_mem_paging_populate(struct domain *d, unsigned long gfn)
     /* Pause domain if request came from guest and gfn has paging type */
     if ( p2m_is_paging(p2mt) && v->domain == d )
     {
-        vcpu_pause_nosync(v);
+        mem_event_vcpu_pause(v);
         req.flags |= MEM_EVENT_FLAG_VCPU_PAUSED;
     }
     /* No need to inform pager if the gfn is not in the page-out path */
@@ -1319,7 +1319,7 @@ void p2m_mem_paging_resume(struct domain *d)
         }
         /* Unpause domain */
         if ( rsp.flags & MEM_EVENT_FLAG_VCPU_PAUSED )
-            vcpu_unpause(v);
+            mem_event_vcpu_unpause(v);
     }
 }
 
@@ -1414,7 +1414,7 @@ bool_t p2m_mem_access_check(paddr_t gpa, bool_t gla_valid, unsigned long gla,
 
     /* Pause the current VCPU */
     if ( p2ma != p2m_access_n2rwx )
-        vcpu_pause_nosync(v);
+        mem_event_vcpu_pause(v);
 
     /* VCPU may be paused, return whether we promoted automatically */
     return (p2ma == p2m_access_n2rwx);
@@ -1440,7 +1440,7 @@ void p2m_mem_access_resume(struct domain *d)
 
         /* Unpause domain */
         if ( rsp.flags & MEM_EVENT_FLAG_VCPU_PAUSED )
-            vcpu_unpause(v);
+            mem_event_vcpu_unpause(v);
     }
 }
 
diff --git a/xen/include/asm-x86/mem_event.h b/xen/include/asm-x86/mem_event.h
index 045ef9b..ed4481a 100644
--- a/xen/include/asm-x86/mem_event.h
+++ b/xen/include/asm-x86/mem_event.h
@@ -66,6 +66,9 @@ int do_mem_event_op(int op, uint32_t domain, void *arg);
 int mem_event_domctl(struct domain *d, xen_domctl_mem_event_op_t *mec,
                      XEN_GUEST_HANDLE_PARAM(void) u_domctl);
 
+void mem_event_vcpu_pause(struct vcpu *v);
+void mem_event_vcpu_unpause(struct vcpu *v);
+
 #endif /* __MEM_EVENT_H__ */
 
 
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 2f876f5..62a4785 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -214,6 +214,9 @@ struct vcpu
     unsigned long    pause_flags;
     atomic_t         pause_count;
 
+    /* VCPU paused for mem_event replies. */
+    atomic_t         mem_event_pause_count;
+
     /* IRQ-safe virq_lock protects against delivering VIRQ to stale evtchn. */
     evtchn_port_t    virq_to_evtchn[NR_VIRQS];
     spinlock_t       virq_lock;
-- 
1.7.10.4