[Qemu-devel] [RFC v3 4/8] pc: fix QEMU crashing when more than ~50 memory hotplugged

[Igor Mammedov <imammedo@redhat.com>]

QEMU asserts in vhost due to hitting vhost backend limit
on number of supported memory regions.

Describe all hotplugged memory as one continuos range
to vhost with linear 1:1 HVA->GPA mapping in backend.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 hw/virtio/vhost.c         | 47 ++++++++++++++++++++++++++++++++++++++++++++---
 include/hw/virtio/vhost.h |  1 +
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 2712c6f..7bc27f0 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -432,6 +432,10 @@ static void vhost_set_memory(MemoryListener *listener,
 
     assert(size);
 
+    if (!dev->rsvd_hva.mr) {
+        dev->rsvd_hva = memory_region_find_hva_range(section->mr);
+    }
+
     /* Optimize no-change case. At least cirrus_vga does this a lot at this time. */
     ram = memory_region_get_ram_ptr(section->mr) + section->offset_within_region;
     if (add) {
@@ -472,6 +476,42 @@ static void vhost_begin(MemoryListener *listener)
     dev->mem_changed_start_addr = -1;
 }
 
+static int vhost_set_mem_table(struct vhost_dev *dev)
+{
+    hwaddr start_addr = 0;
+    ram_addr_t size = 0;
+    struct vhost_memory *mem;
+    int r, i;
+
+    /* drop memory ranges from continuos HVA */
+    mem = g_memdup(dev->mem, offsetof(struct vhost_memory, regions) +
+                       dev->mem->nregions * sizeof dev->mem->regions[0]);
+    start_addr = dev->rsvd_hva.offset_within_address_space;
+    size = int128_get64(dev->rsvd_hva.size);
+    for (i = 0; i < mem->nregions; i++) {
+        if (mem->regions[i].guest_phys_addr >= start_addr &&
+            mem->regions[i].guest_phys_addr < start_addr + size) {
+            mem->nregions--;
+            memmove(&mem->regions[i], &mem->regions[i + 1],
+                    (mem->nregions - i) * sizeof mem->regions[0]);
+        }
+    }
+    /* add one continuos HVA entry if memory ranges from it is present */
+    if (dev->mem->nregions > mem->nregions) {
+        struct vhost_memory_region *reg = &mem->regions[mem->nregions];
+
+        reg->guest_phys_addr = start_addr;
+        reg->memory_size = size;
+        reg->userspace_addr =
+            (__u64)memory_region_get_ram_ptr(dev->rsvd_hva.mr);
+        mem->nregions++;
+    }
+
+    r = dev->vhost_ops->vhost_call(dev, VHOST_SET_MEM_TABLE, mem);
+    g_free(mem);
+    return r;
+}
+
 static void vhost_commit(MemoryListener *listener)
 {
     struct vhost_dev *dev = container_of(listener, struct vhost_dev,
@@ -500,7 +540,7 @@ static void vhost_commit(MemoryListener *listener)
     }
 
     if (!dev->log_enabled) {
-        r = dev->vhost_ops->vhost_call(dev, VHOST_SET_MEM_TABLE, dev->mem);
+        r = vhost_set_mem_table(dev);
         assert(r >= 0);
         dev->memory_changed = false;
         return;
@@ -513,7 +553,7 @@ static void vhost_commit(MemoryListener *listener)
     if (dev->log_size < log_size) {
         vhost_dev_log_resize(dev, log_size + VHOST_LOG_BUFFER);
     }
-    r = dev->vhost_ops->vhost_call(dev, VHOST_SET_MEM_TABLE, dev->mem);
+    r = vhost_set_mem_table(dev);
     assert(r >= 0);
     /* To log less, can only decrease log size after table update. */
     if (dev->log_size > log_size + VHOST_LOG_BUFFER) {
@@ -956,6 +996,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
         migrate_add_blocker(hdev->migration_blocker);
     }
     hdev->mem = g_malloc0(offsetof(struct vhost_memory, regions));
+    memset(&hdev->rsvd_hva, 0, sizeof hdev->rsvd_hva);
     hdev->n_mem_sections = 0;
     hdev->mem_sections = NULL;
     hdev->log = NULL;
@@ -1119,7 +1160,7 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
     if (r < 0) {
         goto fail_features;
     }
-    r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_MEM_TABLE, hdev->mem);
+    r = vhost_set_mem_table(hdev);
     if (r < 0) {
         r = -errno;
         goto fail_mem;
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index dd51050..d41bf2f 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -40,6 +40,7 @@ struct vhost_dev {
     struct vhost_memory *mem;
     int n_mem_sections;
     MemoryRegionSection *mem_sections;
+    MemoryRegionSection rsvd_hva;
     struct vhost_virtqueue *vqs;
     int nvqs;
     /* the first virtqueue which would be used by this vhost dev */
-- 
1.8.3.1