[RFC PATCH 2/3] KVM: keep track of running vcpu ioctls

[Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>]

Just as in kvm-all.c, introduce per-vcpu QemuLockCnt to keep
track when vcpus are executing an ioctl.
Keep a per-vcpu lock to minimize cache line bouncing.
The kvm listener will then use the lock to prevent new ioctls
from running when modifying memslots.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>
---
 accel/kvm/kvm-all.c   | 14 ++++++++++++++
 hw/core/cpu-common.c  |  2 ++
 include/hw/core/cpu.h |  3 +++
 3 files changed, 19 insertions(+)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 099d7bda56..48cdf1fecd 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2811,6 +2811,18 @@ static void kvm_eat_signals(CPUState *cpu)
     } while (sigismember(&chkset, SIG_IPI));
 }
 
+static void kvm_cpu_set_in_ioctl(CPUState *cpu, bool in_ioctl)
+{
+    if (unlikely(qemu_mutex_iothread_locked())) {
+        return;
+    }
+    if (in_ioctl) {
+        qemu_lockcnt_inc(&cpu->in_ioctl_lock);
+    } else {
+        qemu_lockcnt_dec(&cpu->in_ioctl_lock);
+    }
+}
+
 static void kvm_set_in_ioctl(bool in_ioctl)
 {
     if (likely(qemu_mutex_iothread_locked())) {
@@ -3049,7 +3061,9 @@ int kvm_vcpu_ioctl(CPUState *cpu, int type, ...)
     va_end(ap);
 
     trace_kvm_vcpu_ioctl(cpu->cpu_index, type, arg);
+    kvm_cpu_set_in_ioctl(cpu, true);
     ret = ioctl(cpu->kvm_fd, type, arg);
+    kvm_cpu_set_in_ioctl(cpu, false);
     if (ret == -1) {
         ret = -errno;
     }
diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c
index f9fdd46b9d..8d6a4b1b65 100644
--- a/hw/core/cpu-common.c
+++ b/hw/core/cpu-common.c
@@ -237,6 +237,7 @@ static void cpu_common_initfn(Object *obj)
     cpu->nr_threads = 1;
 
     qemu_mutex_init(&cpu->work_mutex);
+    qemu_lockcnt_init(&cpu->in_ioctl_lock);
     QSIMPLEQ_INIT(&cpu->work_list);
     QTAILQ_INIT(&cpu->breakpoints);
     QTAILQ_INIT(&cpu->watchpoints);
@@ -248,6 +249,7 @@ static void cpu_common_finalize(Object *obj)
 {
     CPUState *cpu = CPU(obj);
 
+    qemu_lockcnt_destroy(&cpu->in_ioctl_lock);
     qemu_mutex_destroy(&cpu->work_mutex);
 }
 
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index f9b58773f7..07852b2a3c 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -397,6 +397,9 @@ struct CPUState {
     uint32_t kvm_fetch_index;
     uint64_t dirty_pages;
 
+    /* kvm only for now: CPU is in kvm_vcpu_ioctl() (esp. KVM_RUN) */
+    QemuLockCnt in_ioctl_lock;
+
     /* Used for events with 'vcpu' and *without* the 'disabled' properties */
     DECLARE_BITMAP(trace_dstate_delayed, CPU_TRACE_DSTATE_MAX_EVENTS);
     DECLARE_BITMAP(trace_dstate, CPU_TRACE_DSTATE_MAX_EVENTS);
-- 
2.31.1