[PATCH RFC 4/4] kvm: Implement atomic memory region resizes via region_resize()

[David Hildenbrand <david@redhat.com>]

virtio-mem wants to resize (esp. grow) ram memory regions while the guest
is already aware of them and makes use of them. Resizing a KVM slot can
only currently be done by removing it and re-adding it. While the kvm slot
is temporarily removed, VCPUs that try to read from these slots will fault.
But also, other ioctls might depend on all slots being in place.

Let's inhibit most KVM ioctls while performing the resize. Once we have an
ioctl that can perform atomic resizes (e.g., KVM_SET_USER_MEMORY_REGION
extensions), we can make inhibiting optional at runtime.

Also, make sure to hold the kvm_slots_lock while performing both
actions (removing+re-adding).

Note: Resizes of memory regions currently seems to happen during bootup
only, so I don't think any existing RT users should be affected.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: kvm@vger.kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 accel/kvm/kvm-all.c   | 121 +++++++++++++++++++++++++++++++++++++++---
 include/hw/core/cpu.h |   3 ++
 2 files changed, 117 insertions(+), 7 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 439a4efe52..bba58db098 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -149,6 +149,21 @@ bool kvm_msi_use_devid;
 static bool kvm_immediate_exit;
 static hwaddr kvm_max_slot_size = ~0;
 
+/*
+ * While holding this lock in write, no new KVM ioctls can be started, but
+ * kvm ioctl inhibitors will have to wait for existing ones to finish
+ * (indicated by cpu->in_ioctl and kvm_in_ioctl, both updated with this lock
+ * held in read when entering the ioctl).
+ */
+pthread_rwlock_t kvm_ioctl_lock;
+/*
+ * Atomic counter of active KVM ioctls except
+ * - The KVM ioctl inhibitor is doing an ioctl
+ * - kvm_ioctl(): Harmless and not interesting for inhibitors.
+ * - kvm_vcpu_ioctl(): Tracked via cpu->in_ioctl.
+ */
+static int kvm_in_ioctl;
+
 static const KVMCapabilityInfo kvm_required_capabilites[] = {
     KVM_CAP_INFO(USER_MEMORY),
     KVM_CAP_INFO(DESTROY_MEMORY_REGION_WORKS),
@@ -1023,6 +1038,7 @@ void kvm_set_max_memslot_size(hwaddr max_slot_size)
     kvm_max_slot_size = max_slot_size;
 }
 
+/* Called with KVMMemoryListener.slots_lock held */
 static void kvm_set_phys_mem(KVMMemoryListener *kml,
                              MemoryRegionSection *section, bool add)
 {
@@ -1052,14 +1068,12 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
     ram = memory_region_get_ram_ptr(mr) + section->offset_within_region +
           (start_addr - section->offset_within_address_space);
 
-    kvm_slots_lock(kml);
-
     if (!add) {
         do {
             slot_size = MIN(kvm_max_slot_size, size);
             mem = kvm_lookup_matching_slot(kml, start_addr, slot_size);
             if (!mem) {
-                goto out;
+                return;
             }
             if (mem->flags & KVM_MEM_LOG_DIRTY_PAGES) {
                 kvm_physical_sync_dirty_bitmap(kml, section);
@@ -1079,7 +1093,7 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
             start_addr += slot_size;
             size -= slot_size;
         } while (size);
-        goto out;
+        return;
     }
 
     /* register the new slot */
@@ -1108,9 +1122,6 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml,
         ram += slot_size;
         size -= slot_size;
     } while (size);
-
-out:
-    kvm_slots_unlock(kml);
 }
 
 static void kvm_region_add(MemoryListener *listener,
@@ -1119,7 +1130,9 @@ static void kvm_region_add(MemoryListener *listener,
     KVMMemoryListener *kml = container_of(listener, KVMMemoryListener, listener);
 
     memory_region_ref(section->mr);
+    kvm_slots_lock(kml);
     kvm_set_phys_mem(kml, section, true);
+    kvm_slots_unlock(kml);
 }
 
 static void kvm_region_del(MemoryListener *listener,
@@ -1127,10 +1140,68 @@ static void kvm_region_del(MemoryListener *listener,
 {
     KVMMemoryListener *kml = container_of(listener, KVMMemoryListener, listener);
 
+    kvm_slots_lock(kml);
     kvm_set_phys_mem(kml, section, false);
+    kvm_slots_unlock(kml);
     memory_region_unref(section->mr);
 }
 
+/*
+ * Certain updates (e.g., resizing memory regions) require temporarily removing
+ * kvm memory slots. Make sure any ioctl sees a consistent memory slot state.
+ */
+static void kvm_ioctl_inhibit_begin(void)
+{
+    CPUState *cpu;
+
+    /*
+     * We allow to inhibit only when holding the BQL, so we can identify
+     * when an inhibitor wants to issue an ioctl easily.
+     */
+    g_assert(qemu_mutex_iothread_locked());
+
+    pthread_rwlock_wrlock(&kvm_ioctl_lock);
+
+    /* Inhibiting happens rarely, we can keep things simple and spin here. */
+    while (true) {
+        bool any_cpu_in_ioctl = false;
+
+        CPU_FOREACH(cpu) {
+            if (atomic_read(&cpu->in_ioctl)) {
+                any_cpu_in_ioctl = true;
+                qemu_cpu_kick(cpu);
+            }
+        }
+        if (!any_cpu_in_ioctl && !atomic_read(&kvm_in_ioctl)) {
+            break;
+        }
+        g_usleep(100);
+    }
+}
+
+static void kvm_ioctl_inhibit_end(void)
+{
+    pthread_rwlock_unlock(&kvm_ioctl_lock);
+}
+
+static void kvm_region_resize(MemoryListener *listener,
+                              MemoryRegionSection *section, Int128 new)
+{
+    KVMMemoryListener *kml = container_of(listener, KVMMemoryListener,
+                                          listener);
+    MemoryRegionSection new_section = *section;
+
+    new_section.size = new;
+
+    kvm_slots_lock(kml);
+    /* Inhibit KVM ioctls while temporarily removing slots. */
+    kvm_ioctl_inhibit_begin();
+    kvm_set_phys_mem(kml, section, false);
+    kvm_set_phys_mem(kml, &new_section, true);
+    kvm_ioctl_inhibit_end();
+    kvm_slots_unlock(kml);
+}
+
 static void kvm_log_sync(MemoryListener *listener,
                          MemoryRegionSection *section)
 {
@@ -1249,6 +1320,7 @@ void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
 
     kml->listener.region_add = kvm_region_add;
     kml->listener.region_del = kvm_region_del;
+    kml->listener.region_resize = kvm_region_resize;
     kml->listener.log_start = kvm_log_start;
     kml->listener.log_stop = kvm_log_stop;
     kml->listener.log_sync = kvm_log_sync;
@@ -1894,6 +1966,7 @@ static int kvm_init(MachineState *ms)
     assert(TARGET_PAGE_SIZE <= qemu_real_host_page_size);
 
     s->sigmask_len = 8;
+    pthread_rwlock_init(&kvm_ioctl_lock, NULL);
 
 #ifdef KVM_CAP_SET_GUEST_DEBUG
     QTAILQ_INIT(&s->kvm_sw_breakpoints);
@@ -2304,6 +2377,34 @@ 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) {
+        pthread_rwlock_rdlock(&kvm_ioctl_lock);
+        atomic_set(&cpu->in_ioctl, true);
+        pthread_rwlock_unlock(&kvm_ioctl_lock);
+    } else {
+        atomic_set(&cpu->in_ioctl, false);
+    }
+}
+
+static void kvm_set_in_ioctl(bool in_ioctl)
+{
+    if (likely(qemu_mutex_iothread_locked())) {
+        return;
+    }
+    if (in_ioctl) {
+        pthread_rwlock_rdlock(&kvm_ioctl_lock);
+        atomic_inc(&kvm_in_ioctl);
+        pthread_rwlock_unlock(&kvm_ioctl_lock);
+    } else {
+        atomic_dec(&kvm_in_ioctl);
+    }
+}
+
 int kvm_cpu_exec(CPUState *cpu)
 {
     struct kvm_run *run = cpu->kvm_run;
@@ -2488,7 +2589,9 @@ int kvm_vm_ioctl(KVMState *s, int type, ...)
     va_end(ap);
 
     trace_kvm_vm_ioctl(type, arg);
+    kvm_set_in_ioctl(true);
     ret = ioctl(s->vmfd, type, arg);
+    kvm_set_in_ioctl(false);
     if (ret == -1) {
         ret = -errno;
     }
@@ -2506,7 +2609,9 @@ 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;
     }
@@ -2524,7 +2629,9 @@ int kvm_device_ioctl(int fd, int type, ...)
     va_end(ap);
 
     trace_kvm_device_ioctl(fd, type, arg);
+    kvm_set_in_ioctl(true);
     ret = ioctl(fd, type, arg);
+    kvm_set_in_ioctl(false);
     if (ret == -1) {
         ret = -errno;
     }
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index 73e9a869a4..4fbff6f3d7 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -431,6 +431,9 @@ struct CPUState {
     /* shared by kvm, hax and hvf */
     bool vcpu_dirty;
 
+    /* kvm only for now: CPU is in kvm_vcpu_ioctl() (esp. KVM_RUN) */
+    bool in_ioctl;
+
     /* Used to keep track of an outstanding cpu throttle thread for migration
      * autoconverge
      */
-- 
2.24.1