[Qemu-devel] [RFC 2/2] pc: fix QEMU crashing when more than ~50 memory hotplugged

[Igor Mammedov <imammedo@redhat.com>]

QEMU assert in vhost due to hitting vhost bachend limit
on number of supported memory regions.

Instead of increasing limit in backends, describe all hotlugged
memory as one continuos range to vhost that implements linear
1:1 HVA->GPA mapping in backend.
It allows to avoid increasing VHOST_MEMORY_MAX_NREGIONS limit
in kernel and refactoring current region lookup algorithm
to a faster/scalable datastructure. The same applies to
vhost user which has even lower limit.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 hw/i386/pc.c      |  4 ++--
 hw/virtio/vhost.c | 15 ++++++++++++---
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 1eb1db0..c722339 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1306,8 +1306,8 @@ FWCfgState *pc_memory_init(MachineState *machine,
             exit(EXIT_FAILURE);
         }
 
-        memory_region_init(&pcms->hotplug_memory, OBJECT(pcms),
-                           "hotplug-memory", hotplug_mem_size);
+        memory_region_init_rsvd_hva(&pcms->hotplug_memory, OBJECT(pcms),
+                                    "hotplug-memory", hotplug_mem_size);
         memory_region_add_subregion(system_memory, pcms->hotplug_memory_base,
                                     &pcms->hotplug_memory);
     }
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 54851b7..44cfaab 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -380,6 +380,7 @@ static void vhost_set_memory(MemoryListener *listener,
     bool log_dirty = memory_region_is_logging(section->mr);
     int s = offsetof(struct vhost_memory, regions) +
         (dev->mem->nregions + 1) * sizeof dev->mem->regions[0];
+    MemoryRegionSection rsvd_hva;
     void *ram;
 
     dev->mem = g_realloc(dev->mem, s);
@@ -388,17 +389,25 @@ static void vhost_set_memory(MemoryListener *listener,
         add = false;
     }
 
+    rsvd_hva = memory_region_find_rsvd_hva(section->mr);
+    if (rsvd_hva.mr) {
+        start_addr = rsvd_hva.offset_within_address_space;
+        size = int128_get64(rsvd_hva.size);
+        ram = memory_region_get_ram_ptr(rsvd_hva.mr);
+    } else {
+        ram = memory_region_get_ram_ptr(section->mr) + section->offset_within_region;
+    }
+
     assert(size);
 
     /* 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) {
-        if (!vhost_dev_cmp_memory(dev, start_addr, size, (uintptr_t)ram)) {
+        if (!rsvd_hva.mr && !vhost_dev_cmp_memory(dev, start_addr, size, (uintptr_t)ram)) {
             /* Region exists with same address. Nothing to do. */
             return;
         }
     } else {
-        if (!vhost_dev_find_reg(dev, start_addr, size)) {
+        if (!rsvd_hva.mr && !vhost_dev_find_reg(dev, start_addr, size)) {
             /* Removing region that we don't access. Nothing to do. */
             return;
         }
-- 
1.8.3.1