vhost: add vhost_has_free_slot() interface

it will allow for other parts of QEMU check if it's safe
to map memory region during hotplug/runtime.
That way hotplug path will have a chance to cancel
hotplug operation instead of crashing in vhost_commit().

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-backend.c

@@ -11,6 +11,7 @@
 #include "hw/virtio/vhost.h"
 #include "hw/virtio/vhost-backend.h"
 #include "qemu/error-report.h"
+#include "linux/vhost.h"
 
 #include <sys/ioctl.h>
 
@@ -49,12 +50,30 @@ static int vhost_kernel_get_vq_index(struct vhost_dev *dev, int idx)
     return idx - dev->vq_index;
 }
 
+static int vhost_kernel_memslots_limit(struct vhost_dev *dev)
+{
+    int limit = 64;
+    char *s;
+
+    if (g_file_get_contents("/sys/module/vhost/parameters/max_mem_regions",
+                            &s, NULL, NULL)) {
+        uint64_t val = g_ascii_strtoull(s, NULL, 10);
+        if (!((val == G_MAXUINT64 || !val) && errno)) {
+            return val;
+        }
+        error_report("ignoring invalid max_mem_regions value in vhost module:"
+                     " %s", s);
+    }
+    return limit;
+}
+
 static const VhostOps kernel_ops = {
         .backend_type = VHOST_BACKEND_TYPE_KERNEL,
         .vhost_call = vhost_kernel_call,
         .vhost_backend_init = vhost_kernel_init,
         .vhost_backend_cleanup = vhost_kernel_cleanup,
         .vhost_backend_get_vq_index = vhost_kernel_get_vq_index,
+        .vhost_backend_memslots_limit = vhost_kernel_memslots_limit,
 };
 
 int vhost_set_backend_type(struct vhost_dev *dev, VhostBackendType backend_type)

hw/virtio/vhost-user.c

@@ -440,6 +440,11 @@ static int vhost_user_get_vq_index(struct vhost_dev *dev, int idx)
     return idx;
 }
 
+static int vhost_user_memslots_limit(struct vhost_dev *dev)
+{
+    return VHOST_MEMORY_MAX_NREGIONS;
+}
+
 const VhostOps user_ops = {
         .backend_type = VHOST_BACKEND_TYPE_USER,
         .vhost_call = vhost_user_call,
@@ -447,4 +452,5 @@ const VhostOps user_ops = {
         .vhost_backend_cleanup = vhost_user_cleanup,
         .vhost_backend_get_vq_index = vhost_user_get_vq_index,
         .vhost_backend_set_vring_enable = vhost_user_set_vring_enable,
+        .vhost_backend_memslots_limit = vhost_user_memslots_limit,
 };

hw/virtio/vhost.c

@@ -26,6 +26,22 @@
 
 static struct vhost_log *vhost_log;
 
+static unsigned int used_memslots;
+static QLIST_HEAD(, vhost_dev) vhost_devices =
+    QLIST_HEAD_INITIALIZER(vhost_devices);
+
+bool vhost_has_free_slot(void)
+{
+    unsigned int slots_limit = ~0U;
+    struct vhost_dev *hdev;
+
+    QLIST_FOREACH(hdev, &vhost_devices, entry) {
+        unsigned int r = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
+        slots_limit = MIN(slots_limit, r);
+    }
+    return slots_limit > used_memslots;
+}
+
 static void vhost_dev_sync_region(struct vhost_dev *dev,
                                   MemoryRegionSection *section,
                                   uint64_t mfirst, uint64_t mlast,
@@ -457,6 +473,7 @@ static void vhost_set_memory(MemoryListener *listener,
     dev->mem_changed_start_addr = MIN(dev->mem_changed_start_addr, start_addr);
     dev->mem_changed_end_addr = MAX(dev->mem_changed_end_addr, start_addr + size - 1);
     dev->memory_changed = true;
+    used_memslots = dev->mem->nregions;
 }
 
 static bool vhost_section(MemoryRegionSection *section)
@@ -916,6 +933,8 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
         return -errno;
     }
 
+    QLIST_INSERT_HEAD(&vhost_devices, hdev, entry);
+
     r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_OWNER, NULL);
     if (r < 0) {
         goto fail;
@@ -972,6 +991,7 @@ fail_vq:
 fail:
     r = -errno;
     hdev->vhost_ops->vhost_backend_cleanup(hdev);
+    QLIST_REMOVE(hdev, entry);
     return r;
 }
 
@@ -989,6 +1009,7 @@ void vhost_dev_cleanup(struct vhost_dev *hdev)
     g_free(hdev->mem);
     g_free(hdev->mem_sections);
     hdev->vhost_ops->vhost_backend_cleanup(hdev);
+    QLIST_REMOVE(hdev, entry);
 }
 
 /* Stop processing guest IO notifications in qemu.

include/hw/virtio/vhost-backend.h

@@ -26,6 +26,7 @@ typedef int (*vhost_backend_init)(struct vhost_dev *dev, void *opaque);
 typedef int (*vhost_backend_cleanup)(struct vhost_dev *dev);
 typedef int (*vhost_backend_get_vq_index)(struct vhost_dev *dev, int idx);
 typedef int (*vhost_backend_set_vring_enable)(struct vhost_dev *dev, int enable);
+typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
 
 typedef struct VhostOps {
     VhostBackendType backend_type;
@@ -34,6 +35,7 @@ typedef struct VhostOps {
     vhost_backend_cleanup vhost_backend_cleanup;
     vhost_backend_get_vq_index vhost_backend_get_vq_index;
     vhost_backend_set_vring_enable vhost_backend_set_vring_enable;
+    vhost_backend_memslots_limit vhost_backend_memslots_limit;
 } VhostOps;
 
 extern const VhostOps user_ops;

include/hw/virtio/vhost.h

@@ -59,6 +59,7 @@ struct vhost_dev {
     const VhostOps *vhost_ops;
     void *opaque;
     struct vhost_log *log;
+    QLIST_ENTRY(vhost_dev) entry;
 };
 
 int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
@@ -83,4 +84,5 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
                             uint64_t features);
 void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
                         uint64_t features);
+bool vhost_has_free_slot(void);
 #endif

stubs/Makefile.objs

@@ -39,3 +39,4 @@ stub-obj-y += cpus.o
 stub-obj-y += kvm.o
 stub-obj-y += qmp_pc_dimm_device_list.o
 stub-obj-y += target-monitor-defs.o
+stub-obj-y += vhost.o

stubs/vhost.c

@@ -0,0 +1,6 @@
+#include "hw/virtio/vhost.h"
+
+bool vhost_has_free_slot(void)
+{
+    return true;
+}