1 files changed, 63 insertions, 0 deletions

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index ab975ff..fb6687d 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -33,6 +33,14 @@
 #include "trace.h"
 
 /*
+ * We only had legacy x86 guests that did not support
+ * VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE. Other targets don't have legacy guests.
+ */
+#if defined(TARGET_X86_64) || defined(TARGET_I386)
+#define VIRTIO_MEM_HAS_LEGACY_GUESTS
+#endif
+
+/*
  * Let's not allow blocks smaller than 1 MiB, for example, to keep the tracking
  * bitmap small.
  */
@@ -110,6 +118,19 @@ static uint64_t virtio_mem_default_block_size(RAMBlock *rb)
     return MAX(page_size, VIRTIO_MEM_MIN_BLOCK_SIZE);
 }
 
+#if defined(VIRTIO_MEM_HAS_LEGACY_GUESTS)
+static bool virtio_mem_has_shared_zeropage(RAMBlock *rb)
+{
+    /*
+     * We only have a guaranteed shared zeropage on ordinary MAP_PRIVATE
+     * anonymous RAM. In any other case, reading unplugged *can* populate a
+     * fresh page, consuming actual memory.
+     */
+    return !qemu_ram_is_shared(rb) && rb->fd < 0 &&
+           qemu_ram_pagesize(rb) == qemu_real_host_page_size;
+}
+#endif /* VIRTIO_MEM_HAS_LEGACY_GUESTS */
+
 /*
  * Size the usable region bigger than the requested size if possible. Esp.
  * Linux guests will only add (aligned) memory blocks in case they fully
@@ -683,15 +704,29 @@ static uint64_t virtio_mem_get_features(VirtIODevice *vdev, uint64_t features,
                                         Error **errp)
 {
     MachineState *ms = MACHINE(qdev_get_machine());
+    VirtIOMEM *vmem = VIRTIO_MEM(vdev);
 
     if (ms->numa_state) {
 #if defined(CONFIG_ACPI)
         virtio_add_feature(&features, VIRTIO_MEM_F_ACPI_PXM);
 #endif
     }
+    assert(vmem->unplugged_inaccessible != ON_OFF_AUTO_AUTO);
+    if (vmem->unplugged_inaccessible == ON_OFF_AUTO_ON) {
+        virtio_add_feature(&features, VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE);
+    }
     return features;
 }
 
+static int virtio_mem_validate_features(VirtIODevice *vdev)
+{
+    if (virtio_host_has_feature(vdev, VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE) &&
+        !virtio_vdev_has_feature(vdev, VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE)) {
+        return -EFAULT;
+    }
+    return 0;
+}
+
 static void virtio_mem_system_reset(void *opaque)
 {
     VirtIOMEM *vmem = VIRTIO_MEM(opaque);
@@ -746,6 +781,29 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
     rb = vmem->memdev->mr.ram_block;
     page_size = qemu_ram_pagesize(rb);
 
+#if defined(VIRTIO_MEM_HAS_LEGACY_GUESTS)
+    switch (vmem->unplugged_inaccessible) {
+    case ON_OFF_AUTO_AUTO:
+        if (virtio_mem_has_shared_zeropage(rb)) {
+            vmem->unplugged_inaccessible = ON_OFF_AUTO_OFF;
+        } else {
+            vmem->unplugged_inaccessible = ON_OFF_AUTO_ON;
+        }
+        break;
+    case ON_OFF_AUTO_OFF:
+        if (!virtio_mem_has_shared_zeropage(rb)) {
+            warn_report("'%s' property set to 'off' with a memdev that does"
+                        " not support the shared zeropage.",
+                        VIRTIO_MEM_UNPLUGGED_INACCESSIBLE_PROP);
+        }
+        break;
+    default:
+        break;
+    }
+#else /* VIRTIO_MEM_HAS_LEGACY_GUESTS */
+    vmem->unplugged_inaccessible = ON_OFF_AUTO_ON;
+#endif /* VIRTIO_MEM_HAS_LEGACY_GUESTS */
+
     /*
      * If the block size wasn't configured by the user, use a sane default. This
      * allows using hugetlbfs backends of any page size without manual
@@ -1141,6 +1199,10 @@ static Property virtio_mem_properties[] = {
     DEFINE_PROP_BOOL(VIRTIO_MEM_PREALLOC_PROP, VirtIOMEM, prealloc, false),
     DEFINE_PROP_LINK(VIRTIO_MEM_MEMDEV_PROP, VirtIOMEM, memdev,
                      TYPE_MEMORY_BACKEND, HostMemoryBackend *),
+#if defined(VIRTIO_MEM_HAS_LEGACY_GUESTS)
+    DEFINE_PROP_ON_OFF_AUTO(VIRTIO_MEM_UNPLUGGED_INACCESSIBLE_PROP, VirtIOMEM,
+                            unplugged_inaccessible, ON_OFF_AUTO_OFF),
+#endif
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -1279,6 +1341,7 @@ static void virtio_mem_class_init(ObjectClass *klass, void *data)
     vdc->unrealize = virtio_mem_device_unrealize;
     vdc->get_config = virtio_mem_get_config;
     vdc->get_features = virtio_mem_get_features;
+    vdc->validate_features = virtio_mem_validate_features;
     vdc->vmsd = &vmstate_virtio_mem_device;
 
     vmc->fill_device_info = virtio_mem_fill_device_info;