[RFC,3/3] kvm: Atomic memslot updates

Commit Message

Emanuele Giuseppe Esposito Nov. 4, 2022, 3:14 p.m. UTC

From: David Hildenbrand <[e-mail]>

If we update an existing memslot (e.g., resize, split), we temporarily
remove the memslot to re-add it immediately afterwards. These updates
are not atomic, especially not for KVM VCPU threads, such that we can
get spurious faults.

Let's inhibit most KVM ioctls while performing relevant updates, such
that we can perform the update just as if it would happen atomically
without additional kernel support.

We capture the add/del changes and apply them in the notifier commit
stage instead. There, we can check for overlaps and perform the ioctl
inhibiting only if really required (-> overlap).

To keep things simple we don't perform additional checks that wouldn't
actually result in an overlap -- such as !RAM memory regions in some
cases (see kvm_set_phys_mem()).

To minimize cache-line bouncing, use a separate indicator
(in_ioctl_lock) per CPU.  Also, make sure to hold the kvm_slots_lock
while performing both actions (removing+re-adding).

We have to wait until all IOCTLs were exited and block new ones from
getting executed. Kick all CPUs, so they will exit the KVM_RUN ioctl.

This approach cannot result in a deadlock as long as the inhibitor does
not hold any locks that might hinder an IOCTL from getting finished and
exited - something fairly unusual. The inhibitor will always hold the BQL.

AFAIKs, one possible candidate would be userfaultfd. If a page cannot be
placed (e.g., during postcopy), because we're waiting for a lock, or if the
userfaultfd thread cannot process a fault, because it is waiting for a
lock, there could be a deadlock. However, the BQL is not applicable here,
because any other guest memory access while holding the BQL would already
result in a deadlock.

Nothing else in the kernel should block forever and wait for userspace
intervention.

Note: pause_all_vcpus()/resume_all_vcpus() or
start_exclusive()/end_exclusive() cannot be used, as they either drop
the BQL or require to be called without the BQL - something inhibitors
cannot handle. We need a low-level locking mechanism that is
deadlock-free even when not releasing the BQL.

Signed-off-by: David Hildenbrand <[e-mail]>
Signed-off-by: Emanuele Giuseppe Esposito <[e-mail]>
Tested-by: Emanuele Giuseppe Esposito <[e-mail]>
---
 accel/kvm/kvm-all.c      | 142 ++++++++++++++++++++++++++++++++++++---
 include/sysemu/kvm_int.h |   8 +++
 2 files changed, 139 insertions(+), 11 deletions(-)

Comments

Paolo Bonzini Nov. 8, 2022, 4:25 p.m. UTC | #1

On 11/4/22 16:14, Emanuele Giuseppe Esposito wrote:
> +    g_assert(qemu_mutex_iothread_locked());

Please add a comment here:

     /* Block further invocations of the ioctls outside the BQL.  */

> +    CPU_FOREACH(cpu) {
> +        qemu_lockcnt_lock(&cpu->in_ioctl_lock);
> +    }
> +    qemu_lockcnt_lock(&kvm_in_ioctl_lock);
>   
> -    kvm_set_phys_mem(kml, section, false);
> -    memory_region_unref(section->mr);
> +    /* Inhibiting happens rarely, we can keep things simple and spin here. */

Not making it spin is pretty easy.  You can add a qemu_event_set to 
kvm_set_in_ioctl() and kvm_cpu_set_in_ioctl(), and here something like:

     if (in_kvm_ioctls()) {
         qemu_event_reset(&kvm_in_ioctl_event);
         if (in_kvm_ioctls()) {
             qemu_event_wait(&kvm_in_ioctl_event);
         }
     }

where in_kvm_ioctls() returns true if any (vCPU or KVM) lockcnt has a 
nonzero count.

Also please create a new header sysemu/accel-blocker.h and 
accel/blocker.c or something like that with all the functions, because 
this code can potentially be used by all KVM-like accelerators.

Paolo

Patch

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 48cdf1fecd..05b7aa5403 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -46,6 +46,7 @@ 
 #include "sysemu/hw_accel.h"
 #include "kvm-cpus.h"
 #include "sysemu/dirtylimit.h"
+#include "qemu/range.h"
 
 #include "hw/boards.h"
 #include "monitor/stats.h"
@@ -1294,6 +1295,7 @@  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)
 {
@@ -1328,14 +1330,12 @@  static void kvm_set_phys_mem(KVMMemoryListener *kml,
     ram = memory_region_get_ram_ptr(mr) + mr_offset;
     ram_start_offset = memory_region_get_ram_addr(mr) + mr_offset;
 
-    kvm_slots_lock();
-
     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) {
                 /*
@@ -1373,7 +1373,7 @@  static void kvm_set_phys_mem(KVMMemoryListener *kml,
             start_addr += slot_size;
             size -= slot_size;
         } while (size);
-        goto out;
+        return;
     }
 
     /* register the new slot */
@@ -1398,9 +1398,6 @@  static void kvm_set_phys_mem(KVMMemoryListener *kml,
         ram += slot_size;
         size -= slot_size;
     } while (size);
-
-out:
-    kvm_slots_unlock();
 }
 
 static void *kvm_dirty_ring_reaper_thread(void *data)
@@ -1457,18 +1454,137 @@  static void kvm_region_add(MemoryListener *listener,
                            MemoryRegionSection *section)
 {
     KVMMemoryListener *kml = container_of(listener, KVMMemoryListener, listener);
+    KVMMemoryUpdate *update;
+
+    update = g_new0(KVMMemoryUpdate, 1);
+    update->section = memory_region_section_new_copy(section);
 
-    memory_region_ref(section->mr);
-    kvm_set_phys_mem(kml, section, true);
+    QSIMPLEQ_INSERT_TAIL(&kml->transaction_add, update, next);
 }
 
 static void kvm_region_del(MemoryListener *listener,
                            MemoryRegionSection *section)
 {
     KVMMemoryListener *kml = container_of(listener, KVMMemoryListener, listener);
+    KVMMemoryUpdate *update;
+
+    update = g_new0(KVMMemoryUpdate, 1);
+    update->section = memory_region_section_new_copy(section);
+
+    QSIMPLEQ_INSERT_TAIL(&kml->transaction_del, update, next);
+}
+
+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());
+
+    CPU_FOREACH(cpu) {
+        qemu_lockcnt_lock(&cpu->in_ioctl_lock);
+    }
+    qemu_lockcnt_lock(&kvm_in_ioctl_lock);
 
-    kvm_set_phys_mem(kml, section, false);
-    memory_region_unref(section->mr);
+    /* Inhibiting happens rarely, we can keep things simple and spin here. */
+    while (true) {
+        bool any_cpu_in_ioctl = false;
+
+        CPU_FOREACH(cpu) {
+            if (qemu_lockcnt_count(&cpu->in_ioctl_lock)) {
+                any_cpu_in_ioctl = true;
+                qemu_cpu_kick(cpu);
+            }
+        }
+        if (!any_cpu_in_ioctl &&
+            !qemu_lockcnt_count(&kvm_in_ioctl_lock)) {
+            break;
+        }
+        g_usleep(100);
+    }
+}
+
+static void kvm_ioctl_inhibit_end(void)
+{
+    CPUState *cpu;
+
+    qemu_lockcnt_unlock(&kvm_in_ioctl_lock);
+    CPU_FOREACH(cpu) {
+        qemu_lockcnt_unlock(&cpu->in_ioctl_lock);
+    }
+}
+
+static void kvm_region_commit(MemoryListener *listener)
+{
+    KVMMemoryListener *kml = container_of(listener, KVMMemoryListener,
+                                          listener);
+    KVMMemoryUpdate *u1, *u2;
+    bool need_inhibit = false;
+
+    if (QSIMPLEQ_EMPTY(&kml->transaction_add) &&
+        QSIMPLEQ_EMPTY(&kml->transaction_del)) {
+        return;
+    }
+
+    /*
+     * We have to be careful when regions to add overlap with ranges to remove.
+     * We have to simulate atomic KVM memslot updates by making sure no ioctl()
+     * is currently active.
+     *
+     * The lists are order by addresses, so it's easy to find overlaps.
+     */
+    u1 = QSIMPLEQ_FIRST(&kml->transaction_del);
+    u2 = QSIMPLEQ_FIRST(&kml->transaction_add);
+    while (u1 && u2) {
+        Range r1, r2;
+
+        range_init_nofail(&r1, u1->section->offset_within_address_space,
+                          int128_get64(u1->section->size));
+        range_init_nofail(&r2, u2->section->offset_within_address_space,
+                          int128_get64(u2->section->size));
+
+        if (range_overlaps_range(&r1, &r2)) {
+            need_inhibit = true;
+            break;
+        }
+        if (range_lob(&r1) < range_lob(&r2)) {
+            u1 = QSIMPLEQ_NEXT(u1, next);
+        } else {
+            u2 = QSIMPLEQ_NEXT(u2, next);
+        }
+    }
+
+
+    kvm_slots_lock();
+    if (need_inhibit) {
+        kvm_ioctl_inhibit_begin();
+    }
+
+    /* Remove all memslots before adding the new ones. */
+    QSIMPLEQ_FOREACH_SAFE(u1, &kml->transaction_del, next, u2) {
+        kvm_set_phys_mem(kml, u1->section, false);
+        memory_region_unref(u1->section->mr);
+
+        QSIMPLEQ_REMOVE(&kml->transaction_del, u1, KVMMemoryUpdate, next);
+        memory_region_section_free_copy(u1->section);
+        g_free(u1);
+    }
+    QSIMPLEQ_FOREACH_SAFE(u1, &kml->transaction_add, next, u2) {
+        memory_region_ref(u1->section->mr);
+        kvm_set_phys_mem(kml, u1->section, true);
+
+        QSIMPLEQ_REMOVE(&kml->transaction_add, u1, KVMMemoryUpdate, next);
+        memory_region_section_free_copy(u1->section);
+        g_free(u1);
+    }
+
+    if (need_inhibit) {
+        kvm_ioctl_inhibit_end();
+    }
+    kvm_slots_unlock();
 }
 
 static void kvm_log_sync(MemoryListener *listener,
@@ -1612,8 +1728,12 @@  void kvm_memory_listener_register(KVMState *s, KVMMemoryListener *kml,
         kml->slots[i].slot = i;
     }
 
+    QSIMPLEQ_INIT(&kml->transaction_add);
+    QSIMPLEQ_INIT(&kml->transaction_del);
+
     kml->listener.region_add = kvm_region_add;
     kml->listener.region_del = kvm_region_del;
+    kml->listener.commit = kvm_region_commit;
     kml->listener.log_start = kvm_log_start;
     kml->listener.log_stop = kvm_log_stop;
     kml->listener.priority = 10;
diff --git a/include/sysemu/kvm_int.h b/include/sysemu/kvm_int.h
index 3b4adcdc10..5ea2d7924b 100644
--- a/include/sysemu/kvm_int.h
+++ b/include/sysemu/kvm_int.h
@@ -12,6 +12,7 @@ 
 #include "exec/memory.h"
 #include "qapi/qapi-types-common.h"
 #include "qemu/accel.h"
+#include "qemu/queue.h"
 #include "sysemu/kvm.h"
 
 typedef struct KVMSlot
@@ -31,10 +32,17 @@  typedef struct KVMSlot
     ram_addr_t ram_start_offset;
 } KVMSlot;
 
+typedef struct KVMMemoryUpdate {
+    QSIMPLEQ_ENTRY(KVMMemoryUpdate) next;
+    MemoryRegionSection *section;
+} KVMMemoryUpdate;
+
 typedef struct KVMMemoryListener {
     MemoryListener listener;
     KVMSlot *slots;
     int as_id;
+    QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_add;
+    QSIMPLEQ_HEAD(, KVMMemoryUpdate) transaction_del;
 } KVMMemoryListener;
 
 #define KVM_MSI_HASHTAB_SIZE    256