diff mbox

[1/2] vhost: add vhost_has_free_slot() interface

Message ID 1437400170-66929-2-git-send-email-imammedo@redhat.com
State New
Headers show

Commit Message

Igor Mammedov July 20, 2015, 1:49 p.m. UTC 
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>
---
 hw/virtio/vhost-backend.c         | 23 ++++++++++++++++++++++-
 hw/virtio/vhost-user.c            |  8 +++++++-
 hw/virtio/vhost.c                 | 21 +++++++++++++++++++++
 include/hw/virtio/vhost-backend.h |  2 ++
 include/hw/virtio/vhost.h         |  1 +
 stubs/Makefile.objs               |  1 +
 stubs/vhost.c                     |  6 ++++++
 7 files changed, 60 insertions(+), 2 deletions(-)
 create mode 100644 stubs/vhost.c

Comments

Michael S. Tsirkin July 29, 2015, 10:20 a.m. UTC | #1 
On Mon, Jul 20, 2015 at 03:49:29PM +0200, Igor Mammedov wrote:
> 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>
> ---
>  hw/virtio/vhost-backend.c         | 23 ++++++++++++++++++++++-
>  hw/virtio/vhost-user.c            |  8 +++++++-
>  hw/virtio/vhost.c                 | 21 +++++++++++++++++++++
>  include/hw/virtio/vhost-backend.h |  2 ++
>  include/hw/virtio/vhost.h         |  1 +
>  stubs/Makefile.objs               |  1 +
>  stubs/vhost.c                     |  6 ++++++
>  7 files changed, 60 insertions(+), 2 deletions(-)
>  create mode 100644 stubs/vhost.c
> 
> diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
> index 4d68a27..46fa707 100644
> --- a/hw/virtio/vhost-backend.c
> +++ b/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>
>  
> @@ -42,11 +43,31 @@ static int vhost_kernel_cleanup(struct vhost_dev *dev)
>      return close(fd);
>  }
>  
> +static int vhost_kernel_memslots_limit(struct vhost_dev *dev)
> +{
> +    int limit;
> +    int s = offsetof(struct vhost_memory, regions);
> +    struct vhost_memory *mem = g_malloc0(s);
> +
> +    assert(dev->mem->nregions);
> +    do {
> +        s += sizeof mem->regions[0];
> +        mem = g_realloc(mem, s);
> +        mem->regions[mem->nregions] = dev->mem->regions[0];
> +        mem->nregions++;
> +    } while (vhost_kernel_call(dev, VHOST_SET_MEM_TABLE, mem) != -1);
> +    limit = mem->nregions - 1 > 0 ? mem->nregions - 1 : 0;
> +    g_free(mem);
> +
> +    return limit;
> +}
> +

I don't like this probing much: if one clocks the size up
significantly, this will slow down the box trying
to allocate kernel memory for no real reason.

I'd rather check /sys/modules/vhost/parameters/max_mem_regions.
If not there, assume 64.

>  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_cleanup = vhost_kernel_cleanup,
> +        .vhost_backend_memslots_limit = vhost_kernel_memslots_limit
>  };
>  
>  int vhost_set_backend_type(struct vhost_dev *dev, VhostBackendType backend_type)
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index d6f2163..0487809 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -352,9 +352,15 @@ static int vhost_user_cleanup(struct vhost_dev *dev)
>      return 0;
>  }
>  
> +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,
>          .vhost_backend_init = vhost_user_init,
> -        .vhost_backend_cleanup = vhost_user_cleanup
> +        .vhost_backend_cleanup = vhost_user_cleanup,
> +        .vhost_backend_memslots_limit = vhost_user_memslots_limit
>          };
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 2712c6f..e964004 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -26,6 +26,18 @@
>  
>  static struct vhost_log *vhost_log;
>  
> +static int used_memslots;
> +static int memslots_limit = -1;
> +
> +bool vhost_has_free_slot(void)
> +{
> +    if (memslots_limit >= 0) {
> +        return memslots_limit > used_memslots;
> +    }
> +
> +    return true;
> +}
> +
>  static void vhost_dev_sync_region(struct vhost_dev *dev,
>                                    MemoryRegionSection *section,
>                                    uint64_t mfirst, uint64_t mlast,
> @@ -457,6 +469,7 @@ 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)
> @@ -1119,6 +1132,14 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
>      if (r < 0) {
>          goto fail_features;
>      }
> +
> +    r = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
> +    if (memslots_limit > 0) {
> +        memslots_limit = MIN(memslots_limit, r);
> +    } else {
> +        memslots_limit = r;
> +    }
> +
>      r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_MEM_TABLE, hdev->mem);
>      if (r < 0) {
>          r = -errno;
> diff --git a/include/hw/virtio/vhost-backend.h b/include/hw/virtio/vhost-backend.h
> index e472f29..28b6714 100644
> --- a/include/hw/virtio/vhost-backend.h
> +++ b/include/hw/virtio/vhost-backend.h
> @@ -24,12 +24,14 @@ typedef int (*vhost_call)(struct vhost_dev *dev, unsigned long int request,
>               void *arg);
>  typedef int (*vhost_backend_init)(struct vhost_dev *dev, void *opaque);
>  typedef int (*vhost_backend_cleanup)(struct vhost_dev *dev);
> +typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
>  
>  typedef struct VhostOps {
>      VhostBackendType backend_type;
>      vhost_call vhost_call;
>      vhost_backend_init vhost_backend_init;
>      vhost_backend_cleanup vhost_backend_cleanup;
> +    vhost_backend_memslots_limit vhost_backend_memslots_limit;
>  } VhostOps;
>  
>  extern const VhostOps user_ops;
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index dd51050..17ff7b6 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -81,4 +81,5 @@ 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
> diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
> index 9937a12..d2f1b21 100644
> --- a/stubs/Makefile.objs
> +++ b/stubs/Makefile.objs
> @@ -38,3 +38,4 @@ stub-obj-$(CONFIG_WIN32) += fd-register.o
>  stub-obj-y += cpus.o
>  stub-obj-y += kvm.o
>  stub-obj-y += qmp_pc_dimm_device_list.o
> +stub-obj-y += vhost.o
> diff --git a/stubs/vhost.c b/stubs/vhost.c
> new file mode 100644
> index 0000000..d346b85
> --- /dev/null
> +++ b/stubs/vhost.c
> @@ -0,0 +1,6 @@
> +#include "hw/virtio/vhost.h"
> +
> +bool vhost_has_free_slot(void)
> +{
> +    return true;
> +}
> -- 
> 1.8.3.1
Igor Mammedov July 29, 2015, 11:26 a.m. UTC | #2 
On Wed, 29 Jul 2015 13:20:24 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Jul 20, 2015 at 03:49:29PM +0200, Igor Mammedov wrote:
> > 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>
> > ---
> >  hw/virtio/vhost-backend.c         | 23 ++++++++++++++++++++++-
> >  hw/virtio/vhost-user.c            |  8 +++++++-
> >  hw/virtio/vhost.c                 | 21 +++++++++++++++++++++
> >  include/hw/virtio/vhost-backend.h |  2 ++
> >  include/hw/virtio/vhost.h         |  1 +
> >  stubs/Makefile.objs               |  1 +
> >  stubs/vhost.c                     |  6 ++++++
> >  7 files changed, 60 insertions(+), 2 deletions(-)
> >  create mode 100644 stubs/vhost.c
> > 
> > diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
> > index 4d68a27..46fa707 100644
> > --- a/hw/virtio/vhost-backend.c
> > +++ b/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>
> >  
> > @@ -42,11 +43,31 @@ static int vhost_kernel_cleanup(struct vhost_dev *dev)
> >      return close(fd);
> >  }
> >  
> > +static int vhost_kernel_memslots_limit(struct vhost_dev *dev)
> > +{
> > +    int limit;
> > +    int s = offsetof(struct vhost_memory, regions);
> > +    struct vhost_memory *mem = g_malloc0(s);
> > +
> > +    assert(dev->mem->nregions);
> > +    do {
> > +        s += sizeof mem->regions[0];
> > +        mem = g_realloc(mem, s);
> > +        mem->regions[mem->nregions] = dev->mem->regions[0];
> > +        mem->nregions++;
> > +    } while (vhost_kernel_call(dev, VHOST_SET_MEM_TABLE, mem) != -1);
> > +    limit = mem->nregions - 1 > 0 ? mem->nregions - 1 : 0;
> > +    g_free(mem);
> > +
> > +    return limit;
> > +}
> > +
> 
> I don't like this probing much: if one clocks the size up
> significantly, this will slow down the box trying
> to allocate kernel memory for no real reason.
> 
> I'd rather check /sys/modules/vhost/parameters/max_mem_regions.
> If not there, assume 64.
I'll respin with this suggestion even if it's a bit ugly.
In addition to it we can add IOCTL interface to detect limit later
when kernel side would be ready for it.


> 
> >  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_cleanup = vhost_kernel_cleanup,
> > +        .vhost_backend_memslots_limit = vhost_kernel_memslots_limit
> >  };
> >  
> >  int vhost_set_backend_type(struct vhost_dev *dev, VhostBackendType backend_type)
> > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > index d6f2163..0487809 100644
> > --- a/hw/virtio/vhost-user.c
> > +++ b/hw/virtio/vhost-user.c
> > @@ -352,9 +352,15 @@ static int vhost_user_cleanup(struct vhost_dev *dev)
> >      return 0;
> >  }
> >  
> > +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,
> >          .vhost_backend_init = vhost_user_init,
> > -        .vhost_backend_cleanup = vhost_user_cleanup
> > +        .vhost_backend_cleanup = vhost_user_cleanup,
> > +        .vhost_backend_memslots_limit = vhost_user_memslots_limit
> >          };
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index 2712c6f..e964004 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -26,6 +26,18 @@
> >  
> >  static struct vhost_log *vhost_log;
> >  
> > +static int used_memslots;
> > +static int memslots_limit = -1;
> > +
> > +bool vhost_has_free_slot(void)
> > +{
> > +    if (memslots_limit >= 0) {
> > +        return memslots_limit > used_memslots;
> > +    }
> > +
> > +    return true;
> > +}
> > +
> >  static void vhost_dev_sync_region(struct vhost_dev *dev,
> >                                    MemoryRegionSection *section,
> >                                    uint64_t mfirst, uint64_t mlast,
> > @@ -457,6 +469,7 @@ 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)
> > @@ -1119,6 +1132,14 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
> >      if (r < 0) {
> >          goto fail_features;
> >      }
> > +
> > +    r = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
> > +    if (memslots_limit > 0) {
> > +        memslots_limit = MIN(memslots_limit, r);
> > +    } else {
> > +        memslots_limit = r;
> > +    }
> > +
> >      r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_MEM_TABLE, hdev->mem);
> >      if (r < 0) {
> >          r = -errno;
> > diff --git a/include/hw/virtio/vhost-backend.h b/include/hw/virtio/vhost-backend.h
> > index e472f29..28b6714 100644
> > --- a/include/hw/virtio/vhost-backend.h
> > +++ b/include/hw/virtio/vhost-backend.h
> > @@ -24,12 +24,14 @@ typedef int (*vhost_call)(struct vhost_dev *dev, unsigned long int request,
> >               void *arg);
> >  typedef int (*vhost_backend_init)(struct vhost_dev *dev, void *opaque);
> >  typedef int (*vhost_backend_cleanup)(struct vhost_dev *dev);
> > +typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
> >  
> >  typedef struct VhostOps {
> >      VhostBackendType backend_type;
> >      vhost_call vhost_call;
> >      vhost_backend_init vhost_backend_init;
> >      vhost_backend_cleanup vhost_backend_cleanup;
> > +    vhost_backend_memslots_limit vhost_backend_memslots_limit;
> >  } VhostOps;
> >  
> >  extern const VhostOps user_ops;
> > diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> > index dd51050..17ff7b6 100644
> > --- a/include/hw/virtio/vhost.h
> > +++ b/include/hw/virtio/vhost.h
> > @@ -81,4 +81,5 @@ 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
> > diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
> > index 9937a12..d2f1b21 100644
> > --- a/stubs/Makefile.objs
> > +++ b/stubs/Makefile.objs
> > @@ -38,3 +38,4 @@ stub-obj-$(CONFIG_WIN32) += fd-register.o
> >  stub-obj-y += cpus.o
> >  stub-obj-y += kvm.o
> >  stub-obj-y += qmp_pc_dimm_device_list.o
> > +stub-obj-y += vhost.o
> > diff --git a/stubs/vhost.c b/stubs/vhost.c
> > new file mode 100644
> > index 0000000..d346b85
> > --- /dev/null
> > +++ b/stubs/vhost.c
> > @@ -0,0 +1,6 @@
> > +#include "hw/virtio/vhost.h"
> > +
> > +bool vhost_has_free_slot(void)
> > +{
> > +    return true;
> > +}
> > -- 
> > 1.8.3.1
>
diff mbox

Patch

diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
index 4d68a27..46fa707 100644
--- a/hw/virtio/vhost-backend.c
+++ b/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>
 
@@ -42,11 +43,31 @@  static int vhost_kernel_cleanup(struct vhost_dev *dev)
     return close(fd);
 }
 
+static int vhost_kernel_memslots_limit(struct vhost_dev *dev)
+{
+    int limit;
+    int s = offsetof(struct vhost_memory, regions);
+    struct vhost_memory *mem = g_malloc0(s);
+
+    assert(dev->mem->nregions);
+    do {
+        s += sizeof mem->regions[0];
+        mem = g_realloc(mem, s);
+        mem->regions[mem->nregions] = dev->mem->regions[0];
+        mem->nregions++;
+    } while (vhost_kernel_call(dev, VHOST_SET_MEM_TABLE, mem) != -1);
+    limit = mem->nregions - 1 > 0 ? mem->nregions - 1 : 0;
+    g_free(mem);
+
+    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_cleanup = vhost_kernel_cleanup,
+        .vhost_backend_memslots_limit = vhost_kernel_memslots_limit
 };
 
 int vhost_set_backend_type(struct vhost_dev *dev, VhostBackendType backend_type)
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index d6f2163..0487809 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -352,9 +352,15 @@  static int vhost_user_cleanup(struct vhost_dev *dev)
     return 0;
 }
 
+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,
         .vhost_backend_init = vhost_user_init,
-        .vhost_backend_cleanup = vhost_user_cleanup
+        .vhost_backend_cleanup = vhost_user_cleanup,
+        .vhost_backend_memslots_limit = vhost_user_memslots_limit
         };
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 2712c6f..e964004 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -26,6 +26,18 @@ 
 
 static struct vhost_log *vhost_log;
 
+static int used_memslots;
+static int memslots_limit = -1;
+
+bool vhost_has_free_slot(void)
+{
+    if (memslots_limit >= 0) {
+        return memslots_limit > used_memslots;
+    }
+
+    return true;
+}
+
 static void vhost_dev_sync_region(struct vhost_dev *dev,
                                   MemoryRegionSection *section,
                                   uint64_t mfirst, uint64_t mlast,
@@ -457,6 +469,7 @@  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)
@@ -1119,6 +1132,14 @@  int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
     if (r < 0) {
         goto fail_features;
     }
+
+    r = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
+    if (memslots_limit > 0) {
+        memslots_limit = MIN(memslots_limit, r);
+    } else {
+        memslots_limit = r;
+    }
+
     r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_MEM_TABLE, hdev->mem);
     if (r < 0) {
         r = -errno;
diff --git a/include/hw/virtio/vhost-backend.h b/include/hw/virtio/vhost-backend.h
index e472f29..28b6714 100644
--- a/include/hw/virtio/vhost-backend.h
+++ b/include/hw/virtio/vhost-backend.h
@@ -24,12 +24,14 @@  typedef int (*vhost_call)(struct vhost_dev *dev, unsigned long int request,
              void *arg);
 typedef int (*vhost_backend_init)(struct vhost_dev *dev, void *opaque);
 typedef int (*vhost_backend_cleanup)(struct vhost_dev *dev);
+typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
 
 typedef struct VhostOps {
     VhostBackendType backend_type;
     vhost_call vhost_call;
     vhost_backend_init vhost_backend_init;
     vhost_backend_cleanup vhost_backend_cleanup;
+    vhost_backend_memslots_limit vhost_backend_memslots_limit;
 } VhostOps;
 
 extern const VhostOps user_ops;
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index dd51050..17ff7b6 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -81,4 +81,5 @@  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
diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
index 9937a12..d2f1b21 100644
--- a/stubs/Makefile.objs
+++ b/stubs/Makefile.objs
@@ -38,3 +38,4 @@  stub-obj-$(CONFIG_WIN32) += fd-register.o
 stub-obj-y += cpus.o
 stub-obj-y += kvm.o
 stub-obj-y += qmp_pc_dimm_device_list.o
+stub-obj-y += vhost.o
diff --git a/stubs/vhost.c b/stubs/vhost.c
new file mode 100644
index 0000000..d346b85
--- /dev/null
+++ b/stubs/vhost.c
@@ -0,0 +1,6 @@ 
+#include "hw/virtio/vhost.h"
+
+bool vhost_has_free_slot(void)
+{
+    return true;
+}