[PATCH v4 8/8] x86/hvm: serialize trap injecting producer and consumer

[Paul Durrant <paul.durrant@citrix.com>]

Since injection works on a remote vCPU, and since there's no
enforcement of the subject vCPU being paused, there's a potential race
between the producing and consuming sides. Fix this by leveraging the
vector field as synchronization variable.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
[re-based]
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
---
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

v3:
- Re-re-re-based after more changes.

v2:
- Re-re-based after Andrew's recent changes.
---
 xen/arch/x86/hvm/dm.c         | 5 ++++-
 xen/arch/x86/hvm/hvm.c        | 8 +++++---
 xen/include/asm-x86/hvm/hvm.h | 3 +++
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/hvm/dm.c b/xen/arch/x86/hvm/dm.c
index 3c031d9..4739deb 100644
--- a/xen/arch/x86/hvm/dm.c
+++ b/xen/arch/x86/hvm/dm.c
@@ -245,13 +245,16 @@ static int inject_trap(struct domain *d, unsigned int vcpuid,
     if ( vcpuid >= d->max_vcpus || !(v = d->vcpu[vcpuid]) )
         return -EINVAL;
 
-    if ( v->arch.hvm_vcpu.inject_trap.vector != -1 )
+    if ( cmpxchg(&v->arch.hvm_vcpu.inject_trap.vector,
+                 HVM_TRAP_VECTOR_UNSET, HVM_TRAP_VECTOR_UPDATING) !=
+         HVM_TRAP_VECTOR_UNSET )
         return -EBUSY;
 
     v->arch.hvm_vcpu.inject_trap.type = type;
     v->arch.hvm_vcpu.inject_trap.insn_len = insn_len;
     v->arch.hvm_vcpu.inject_trap.error_code = error_code;
     v->arch.hvm_vcpu.inject_trap.cr2 = cr2;
+    smp_wmb();
     v->arch.hvm_vcpu.inject_trap.vector = vector;
 
     return 0;
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index f1d59b2..eafad65 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -539,12 +539,14 @@ void hvm_do_resume(struct vcpu *v)
     }
 
     /* Inject pending hw/sw trap */
-    if ( v->arch.hvm_vcpu.inject_trap.vector != -1 )
+    if ( v->arch.hvm_vcpu.inject_trap.vector >= 0 )
     {
+        smp_rmb();
+
         if ( !hvm_event_pending(v) )
             hvm_inject_event(&v->arch.hvm_vcpu.inject_trap);
 
-        v->arch.hvm_vcpu.inject_trap.vector = -1;
+        v->arch.hvm_vcpu.inject_trap.vector = HVM_TRAP_VECTOR_UNSET;
     }
 
     if ( unlikely(v->arch.vm_event) && v->arch.monitor.next_interrupt_enabled )
@@ -1515,7 +1517,7 @@ int hvm_vcpu_initialise(struct vcpu *v)
         (void(*)(unsigned long))hvm_assert_evtchn_irq,
         (unsigned long)v);
 
-    v->arch.hvm_vcpu.inject_trap.vector = -1;
+    v->arch.hvm_vcpu.inject_trap.vector = HVM_TRAP_VECTOR_UNSET;
 
     if ( is_pvh_domain(d) )
     {
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index 04e67fe..9b58346 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -77,6 +77,9 @@ enum hvm_intblk {
 #define HVM_HAP_SUPERPAGE_2MB   0x00000001
 #define HVM_HAP_SUPERPAGE_1GB   0x00000002
 
+#define HVM_TRAP_VECTOR_UNSET    (-1)
+#define HVM_TRAP_VECTOR_UPDATING (-2)
+
 /*
  * The hardware virtual machine (HVM) interface abstracts away from the
  * x86/x86_64 CPU virtualization assist specifics. Currently this interface
-- 
2.1.4


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