kvm: Shutdown KVM and disconnect performance counters on fork

We can't/shouldn't use KVM after a fork since the child and parent
probably point to the same VM. Knowing the exact effects of this is
hard, but they are likely to be messy. We also disconnect the
performance counters attached to the guest. This works around what
seems to be a kernel bug where spurious SIGIOs get delivered to the
forked child process.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>
[sascha.bischoff@arm.com: Rebased patches onto a newer gem5 version]
Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
[andreas.sandberg@arm.com: Fatal if entering KVM in child process ]
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/cpu/kvm/base.cc

@@ -125,7 +125,7 @@ BaseKvmCPU::startup()
     const BaseKvmCPUParams * const p(
         dynamic_cast<const BaseKvmCPUParams *>(params()));
 
-    Kvm &kvm(vm.kvm);
+    Kvm &kvm(*vm.kvm);
 
     BaseCPU::startup();
 
@@ -362,6 +362,29 @@ BaseKvmCPU::drainResume()
     }
 }
 
+void
+BaseKvmCPU::notifyFork()
+{
+    // We should have drained prior to forking, which means that the
+    // tick event shouldn't be scheduled and the CPU is idle.
+    assert(!tickEvent.scheduled());
+    assert(_status == Idle);
+
+    if (vcpuFD != -1) {
+        if (close(vcpuFD) == -1)
+            warn("kvm CPU: notifyFork failed to close vcpuFD\n");
+
+        if (_kvmRun)
+            munmap(_kvmRun, vcpuMMapSize);
+
+        vcpuFD = -1;
+        _kvmRun = NULL;
+
+        hwInstructions.detach();
+        hwCycles.detach();
+    }
+}
+
 void
 BaseKvmCPU::switchOut()
 {
@@ -617,6 +640,9 @@ Tick
 BaseKvmCPU::kvmRun(Tick ticks)
 {
     Tick ticksExecuted;
+    fatal_if(vcpuFD == -1,
+             "Trying to run a KVM CPU in a forked child process. "
+             "This is not supported.\n");
     DPRINTF(KvmRun, "KVM: Executing for %i ticks\n", ticks);
 
     if (ticks == 0) {

src/cpu/kvm/base.hh

@@ -89,6 +89,7 @@ class BaseKvmCPU : public BaseCPU
 
     DrainState drain() override;
     void drainResume() override;
+    void notifyFork() override;
 
     void switchOut() override;
     void takeOverFrom(BaseCPU *cpu) override;

src/cpu/kvm/vm.cc

@@ -291,12 +291,12 @@ Kvm::createVM()
 
 KvmVM::KvmVM(KvmVMParams *params)
     : SimObject(params),
-      kvm(), system(params->system),
-      vmFD(kvm.createVM()),
+      kvm(new Kvm()), system(params->system),
+      vmFD(kvm->createVM()),
       started(false),
       nextVCPUID(0)
 {
-    maxMemorySlot = kvm.capNumMemSlots();
+    maxMemorySlot = kvm->capNumMemSlots();
     /* If we couldn't determine how memory slots there are, guess 32. */
     if (!maxMemorySlot)
         maxMemorySlot = 32;
@@ -307,7 +307,25 @@ KvmVM::KvmVM(KvmVMParams *params)
 
 KvmVM::~KvmVM()
 {
-    close(vmFD);
+    if (vmFD != -1)
+        close(vmFD);
+
+    if (kvm)
+        delete kvm;
+}
+
+void
+KvmVM::notifyFork()
+{
+    if (vmFD != -1) {
+        if (close(vmFD) == -1)
+            warn("kvm VM: notifyFork failed to close vmFD\n");
+
+        vmFD = -1;
+
+        delete kvm;
+        kvm = NULL;
+    }
 }
 
 void

src/cpu/kvm/vm.hh

@@ -295,6 +295,8 @@ class KvmVM : public SimObject
     KvmVM(KvmVMParams *params);
     virtual ~KvmVM();
 
+    void notifyFork();
+
     /**
      * Setup a shared three-page memory region used by the internals
      * of KVM. This is currently only needed by x86 implementations.
@@ -396,7 +398,7 @@ class KvmVM : public SimObject
     int createDevice(uint32_t type, uint32_t flags = 0);
 
     /** Global KVM interface */
-    Kvm kvm;
+    Kvm *kvm;
 
 #if defined(__aarch64__)
   public: // ARM-specific
@@ -504,7 +506,7 @@ class KvmVM : public SimObject
     System *system;
 
     /** KVM VM file descriptor */
-    const int vmFD;
+    int vmFD;
 
     /** Has delayedStartup() already been called? */
     bool started;

src/cpu/kvm/x86_cpu.cc

@@ -519,7 +519,7 @@ X86KvmCPU::X86KvmCPU(X86KvmCPUParams *params)
     : BaseKvmCPU(params),
       useXSave(params->useXSave)
 {
-    Kvm &kvm(vm.kvm);
+    Kvm &kvm(*vm.kvm);
 
     if (!kvm.capSetTSSAddress())
         panic("KVM: Missing capability (KVM_CAP_SET_TSS_ADDR)\n");
@@ -649,7 +649,7 @@ X86KvmCPU::dumpVCpuEvents() const
 void
 X86KvmCPU::dumpMSRs() const
 {
-    const Kvm::MSRIndexVector &supported_msrs(vm.kvm.getSupportedMSRs());
+    const Kvm::MSRIndexVector &supported_msrs(vm.kvm->getSupportedMSRs());
     std::unique_ptr<struct kvm_msrs> msrs(
         newVarStruct<struct kvm_msrs, struct kvm_msr_entry>(
             supported_msrs.size()));
@@ -1539,7 +1539,7 @@ const Kvm::MSRIndexVector &
 X86KvmCPU::getMsrIntersection() const
 {
     if (cachedMsrIntersection.empty()) {
-        const Kvm::MSRIndexVector &kvm_msrs(vm.kvm.getSupportedMSRs());
+        const Kvm::MSRIndexVector &kvm_msrs(vm.kvm->getSupportedMSRs());
 
         DPRINTF(Kvm, "kvm-x86: Updating MSR intersection\n");
         for (auto it = kvm_msrs.cbegin(); it != kvm_msrs.cend(); ++it) {