KVM: use KVM_CAP_IMMEDIATE_EXIT

The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick"
a VCPU out of KVM_RUN through a POSIX signal.  A signal is attached
to a dummy signal handler; by blocking the signal outside KVM_RUN and
unblocking it inside, this possible race is closed:

          VCPU thread                     service thread
   --------------------------------------------------------------
        check flag
                                          set flag
                                          raise signal
        (signal handler does nothing)
        KVM_RUN

However, one issue with KVM_SET_SIGNAL_MASK is that it has to take
tsk->sighand->siglock on every KVM_RUN.  This lock is often on a
remote NUMA node, because it is on the node of a thread's creator.
Taking this lock can be very expensive if there are many userspace
exits (as is the case for SMP Windows VMs without Hyper-V reference
time counter).

KVM_CAP_IMMEDIATE_EXIT provides an alternative, where the flag is
placed directly in kvm_run so that KVM can see it:

          VCPU thread                     service thread
   --------------------------------------------------------------
                                          raise signal
        signal handler
          set run->immediate_exit
        KVM_RUN
          check run->immediate_exit

The previous patches changed QEMU so that the only blocked signal is
SIG_IPI, so we can now stop using KVM_SET_SIGNAL_MASK and sigtimedwait
if KVM_CAP_IMMEDIATE_EXIT is available.

On a 14-VCPU guest, an "inl" operation goes down from 30k to 6k on
an unlocked (no BQL) MemoryRegion, or from 30k to 15k if the BQL
is involved.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

kvm-all.c

@@ -120,6 +120,7 @@ bool kvm_vm_attributes_allowed;
 bool kvm_direct_msi_allowed;
 bool kvm_ioeventfd_any_length_allowed;
 bool kvm_msi_use_devid;
+static bool kvm_immediate_exit;
 
 static const KVMCapabilityInfo kvm_required_capabilites[] = {
     KVM_CAP_INFO(USER_MEMORY),
@@ -1619,6 +1620,7 @@ static int kvm_init(MachineState *ms)
         goto err;
     }
 
+    kvm_immediate_exit = kvm_check_extension(s, KVM_CAP_IMMEDIATE_EXIT);
     s->nr_slots = kvm_check_extension(s, KVM_CAP_NR_MEMSLOTS);
 
     /* If unspecified, use the default value */
@@ -1899,6 +1901,20 @@ static __thread int pending_sigbus_code;
 static __thread bool have_sigbus_pending;
 #endif
 
+static void kvm_cpu_kick(CPUState *cpu)
+{
+    atomic_set(&cpu->kvm_run->immediate_exit, 1);
+}
+
+static void kvm_cpu_kick_self(void)
+{
+    if (kvm_immediate_exit) {
+        kvm_cpu_kick(current_cpu);
+    } else {
+        qemu_cpu_kick_self();
+    }
+}
+
 static void kvm_eat_signals(CPUState *cpu)
 {
     struct timespec ts = { 0, 0 };
@@ -1907,6 +1923,15 @@ static void kvm_eat_signals(CPUState *cpu)
     sigset_t chkset;
     int r;
 
+    if (kvm_immediate_exit) {
+        atomic_set(&cpu->kvm_run->immediate_exit, 0);
+        /* Write kvm_run->immediate_exit before the cpu->exit_request
+         * write in kvm_cpu_exec.
+         */
+        smp_wmb();
+        return;
+    }
+
     sigemptyset(&waitset);
     sigaddset(&waitset, SIG_IPI);
 
@@ -1955,9 +1980,14 @@ int kvm_cpu_exec(CPUState *cpu)
              * instruction emulation. This self-signal will ensure that we
              * leave ASAP again.
              */
-            qemu_cpu_kick_self();
+            kvm_cpu_kick_self();
         }
 
+        /* Read cpu->exit_request before KVM_RUN reads run->immediate_exit.
+         * Matching barrier in kvm_eat_signals.
+         */
+        smp_rmb();
+
         run_ret = kvm_vcpu_ioctl(cpu, KVM_RUN, 0);
 
         attrs = kvm_arch_post_run(cpu, run);
@@ -2431,8 +2461,12 @@ static int kvm_set_signal_mask(CPUState *cpu, const sigset_t *sigset)
     return r;
 }
 
-static void dummy_signal(int sig)
+static void kvm_ipi_signal(int sig)
 {
+    if (current_cpu) {
+        assert(kvm_immediate_exit);
+        kvm_cpu_kick(current_cpu);
+    }
 }
 
 void kvm_init_cpu_signals(CPUState *cpu)
@@ -2442,7 +2476,7 @@ void kvm_init_cpu_signals(CPUState *cpu)
     struct sigaction sigact;
 
     memset(&sigact, 0, sizeof(sigact));
-    sigact.sa_handler = dummy_signal;
+    sigact.sa_handler = kvm_ipi_signal;
     sigaction(SIG_IPI, &sigact, NULL);
 
     pthread_sigmask(SIG_BLOCK, NULL, &set);
@@ -2451,7 +2485,11 @@ void kvm_init_cpu_signals(CPUState *cpu)
     pthread_sigmask(SIG_SETMASK, &set, NULL);
 #endif
     sigdelset(&set, SIG_IPI);
-    r = kvm_set_signal_mask(cpu, &set);
+    if (kvm_immediate_exit) {
+        r = pthread_sigmask(SIG_SETMASK, &set, NULL);
+    } else {
+        r = kvm_set_signal_mask(cpu, &set);
+    }
     if (r) {
         fprintf(stderr, "kvm_set_signal_mask: %s\n", strerror(-r));
         exit(1);