kvm: Add support for multi-system simulation

The introduction of parallel event queues added most of the support
needed to run multiple VMs (systems) within the same gem5
instance. This changeset fixes up signal delivery so that KVM's
control signals are delivered to the thread that executes the CPU's
event queue. Specifically:

  * Timers and counters are now initialized from a separate method
    (startupThread) that is scheduled as the first event in the
    thread-specific event queue. This ensures that they are
    initialized from the thread that is going to execute the CPUs
    event queue and enables signal delivery to the right thread when
    exiting from KVM.

  * The POSIX-timer-based KVM timer (used to force exits from KVM) has
    been updated to deliver signals to the thread that's executing KVM
    instead of the process (thread is undefined in that case). This
    assumes that the timer is instantiated from the thread that is
    going to execute the KVM vCPU.

  * Signal masking is now done using pthread_sigmask instead of
    sigprocmask. The behavior of the latter is undefined in threaded
    applications.

  * Since signal masks can be inherited, make sure to actively unmask
    the control signals when setting up the KVM signal mask.

There are currently no facilities to multiplex between multiple KVM
CPUs in the same event queue, we are therefore limited to
configurations where there is only one KVM CPU per event queue. In
practice, this means that multi-system configurations can be
simulated, but not multiple CPUs in a shared-memory configuration.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/cpu/kvm/base.hh b/src/cpu/kvm/base.hh
index 8788d6c8a..5a0b80b15 100644
--- a/src/cpu/kvm/base.hh
+++ b/src/cpu/kvm/base.hh
@@ -618,6 +618,20 @@ class BaseKvmCPU : public BaseCPU
      */
     bool discardPendingSignal(int signum) const;
 
+    /**
+     * Thread-specific initialization.
+     *
+     * Some KVM-related initialization requires us to know the TID of
+     * the thread that is going to execute our event queue. For
+     * example, when setting up timers, we need to know the TID of the
+     * thread executing in KVM in order to deliver the timer signal to
+     * that thread. This method is called as the first event in this
+     * SimObject's event queue.
+     *
+     * @see startup
+     */
+    void startupThread();
+
     /** Try to drain the CPU if a drain is pending */
     bool tryDrain();