Re: [PATCH for-7.2] vhost: enable vrings in vhost_dev_start() for vhost-user devices

[Stefano Garzarella]

On Thu, Nov 24, 2022 at 01:36:29PM -0500, Stefan Hajnoczi wrote:
On Wed, Nov 23, 2022 at 02:16:30PM +0100, Stefano Garzarella wrote:
Commit 02b61f38d3 ("hw/virtio: incorporate backend features in features")
properly negotiates VHOST_USER_F_PROTOCOL_FEATURES with the vhost-user
backend, but we forgot to enable vrings as specified in
docs/interop/vhost-user.rst:
    If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
    ring starts directly in the enabled state.

    If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
    initialized in a disabled state and is enabled by
    ``VHOST_USER_SET_VRING_ENABLE`` with parameter 1.

Some vhost-user front-ends already did this by calling
vhost_ops.vhost_set_vring_enable() directly:
- backends/cryptodev-vhost.c
- hw/net/virtio-net.c
- hw/virtio/vhost-user-gpio.c

But most didn't do that, so we would leave the vrings disabled and some
backends would not work. We observed this issue with the rust version of
virtiofsd [1], which uses the event loop [2] provided by the
vhost-user-backend crate where requests are not processed if vring is
not enabled.

Let's fix this issue by enabling the vrings in vhost_dev_start() for
vhost-user front-ends that don't already do this directly. Same thing
also in vhost_dev_stop() where we disable vrings.

[1] https://gitlab.com/virtio-fs/virtiofsd
[2] 
https://github.com/rust-vmm/vhost/blob/240fc2966/crates/vhost-user-backend/src/event_loop.rs#L217

Fixes: 02b61f38d3 ("hw/virtio: incorporate backend features in features")
Reported-by: German Maglione <gmagli...@redhat.com>
Tested-by: German Maglione <gmagli...@redhat.com>
Signed-off-by: Stefano Garzarella <sgarz...@redhat.com>
---
 include/hw/virtio/vhost.h      |  6 +++--
 backends/cryptodev-vhost.c     |  4 ++--
 backends/vhost-user.c          |  4 ++--
 hw/block/vhost-user-blk.c      |  4 ++--
 hw/net/vhost_net.c             |  8 +++----
 hw/scsi/vhost-scsi-common.c    |  4 ++--
 hw/virtio/vhost-user-fs.c      |  4 ++--
 hw/virtio/vhost-user-gpio.c    |  4 ++--
 hw/virtio/vhost-user-i2c.c     |  4 ++--
 hw/virtio/vhost-user-rng.c     |  4 ++--
 hw/virtio/vhost-vsock-common.c |  4 ++--
 hw/virtio/vhost.c              | 44 ++++++++++++++++++++++++++++++----
 hw/virtio/trace-events         |  4 ++--
 13 files changed, 67 insertions(+), 31 deletions(-)

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 353252ac3e..67a6807fac 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -184,24 +184,26 @@ static inline bool vhost_dev_is_started(struct vhost_dev 
*hdev)
  * vhost_dev_start() - start the vhost device
  * @hdev: common vhost_dev structure
  * @vdev: the VirtIODevice structure
+ * @vrings: true to have vrings enabled in this call
  *
  * Starts the vhost device. From this point VirtIO feature negotiation
  * can start and the device can start processing VirtIO transactions.
  *
  * Return: 0 on success, < 0 on error.
  */
-int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
+int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings);

 /**
  * vhost_dev_stop() - stop the vhost device
  * @hdev: common vhost_dev structure
  * @vdev: the VirtIODevice structure
+ * @vrings: true to have vrings disabled in this call
  *
  * Stop the vhost device. After the device is stopped the notifiers
  * can be disabled (@vhost_dev_disable_notifiers) and the device can
  * be torn down (@vhost_dev_cleanup).
  */
-void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);
+void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings);

 /**
  * DOC: vhost device configuration handling
diff --git a/backends/cryptodev-vhost.c b/backends/cryptodev-vhost.c
index bc13e466b4..572f87b3be 100644
--- a/backends/cryptodev-vhost.c
+++ b/backends/cryptodev-vhost.c
@@ -94,7 +94,7 @@ cryptodev_vhost_start_one(CryptoDevBackendVhost *crypto,
         goto fail_notifiers;
     }

-    r = vhost_dev_start(&crypto->dev, dev);
+    r = vhost_dev_start(&crypto->dev, dev, false);
     if (r < 0) {
         goto fail_start;
     }
@@ -111,7 +111,7 @@ static void
 cryptodev_vhost_stop_one(CryptoDevBackendVhost *crypto,
                                  VirtIODevice *dev)
 {
-    vhost_dev_stop(&crypto->dev, dev);
+    vhost_dev_stop(&crypto->dev, dev, false);
     vhost_dev_disable_notifiers(&crypto->dev, dev);
 }

diff --git a/backends/vhost-user.c b/backends/vhost-user.c
index 5dedb2d987..7bfcaef976 100644
--- a/backends/vhost-user.c
+++ b/backends/vhost-user.c
@@ -85,7 +85,7 @@ vhost_user_backend_start(VhostUserBackend *b)
     }

     b->dev.acked_features = b->vdev->guest_features;
-    ret = vhost_dev_start(&b->dev, b->vdev);
+    ret = vhost_dev_start(&b->dev, b->vdev, true);
     if (ret < 0) {
         error_report("Error start vhost dev");
         goto err_guest_notifiers;
@@ -120,7 +120,7 @@ vhost_user_backend_stop(VhostUserBackend *b)
         return;
     }

-    vhost_dev_stop(&b->dev, b->vdev);
+    vhost_dev_stop(&b->dev, b->vdev, true);

     if (k->set_guest_notifiers) {
         ret = k->set_guest_notifiers(qbus->parent,
diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index 0d5190accf..1177064631 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -178,7 +178,7 @@ static int vhost_user_blk_start(VirtIODevice *vdev, Error 
**errp)
     }

     s->dev.vq_index_end = s->dev.nvqs;
-    ret = vhost_dev_start(&s->dev, vdev);
+    ret = vhost_dev_start(&s->dev, vdev, true);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Error starting vhost");
         goto err_guest_notifiers;
@@ -213,7 +213,7 @@ static void vhost_user_blk_stop(VirtIODevice *vdev)
         return;
     }

-    vhost_dev_stop(&s->dev, vdev);
+    vhost_dev_stop(&s->dev, vdev, true);

     ret = k->set_guest_notifiers(qbus->parent, s->dev.nvqs, false);
     if (ret < 0) {
diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
index 26e4930676..043058ff43 100644
--- a/hw/net/vhost_net.c
+++ b/hw/net/vhost_net.c
@@ -259,7 +259,7 @@ static int vhost_net_start_one(struct vhost_net *net,
         goto fail_notifiers;
     }

-    r = vhost_dev_start(&net->dev, dev);
+    r = vhost_dev_start(&net->dev, dev, false);
     if (r < 0) {
         goto fail_start;
     }
@@ -308,7 +308,7 @@ fail:
     if (net->nc->info->poll) {
         net->nc->info->poll(net->nc, true);
     }
-    vhost_dev_stop(&net->dev, dev);
+    vhost_dev_stop(&net->dev, dev, false);
 fail_start:
     vhost_dev_disable_notifiers(&net->dev, dev);
 fail_notifiers:
@@ -329,7 +329,7 @@ static void vhost_net_stop_one(struct vhost_net *net,
     if (net->nc->info->poll) {
         net->nc->info->poll(net->nc, true);
     }
-    vhost_dev_stop(&net->dev, dev);
+    vhost_dev_stop(&net->dev, dev, false);
     if (net->nc->info->stop) {
         net->nc->info->stop(net->nc);
     }
@@ -606,7 +606,7 @@ err_start:
         assert(r >= 0);
     }

-    vhost_dev_stop(&net->dev, vdev);
+    vhost_dev_stop(&net->dev, vdev, false);

     return r;
 }
diff --git a/hw/scsi/vhost-scsi-common.c b/hw/scsi/vhost-scsi-common.c
index 767f827e55..18ea5dcfa1 100644
--- a/hw/scsi/vhost-scsi-common.c
+++ b/hw/scsi/vhost-scsi-common.c
@@ -68,7 +68,7 @@ int vhost_scsi_common_start(VHostSCSICommon *vsc)
         goto err_guest_notifiers;
     }

-    ret = vhost_dev_start(&vsc->dev, vdev);
+    ret = vhost_dev_start(&vsc->dev, vdev, true);
     if (ret < 0) {
         error_report("Error start vhost dev");
         goto err_guest_notifiers;
@@ -101,7 +101,7 @@ void vhost_scsi_common_stop(VHostSCSICommon *vsc)
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
     int ret = 0;

-    vhost_dev_stop(&vsc->dev, vdev);
+    vhost_dev_stop(&vsc->dev, vdev, true);

     if (k->set_guest_notifiers) {
         ret = k->set_guest_notifiers(qbus->parent, vsc->dev.nvqs, false);
diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
index dc4014cdef..d97b179e6f 100644
--- a/hw/virtio/vhost-user-fs.c
+++ b/hw/virtio/vhost-user-fs.c
@@ -76,7 +76,7 @@ static void vuf_start(VirtIODevice *vdev)
     }

     fs->vhost_dev.acked_features = vdev->guest_features;
-    ret = vhost_dev_start(&fs->vhost_dev, vdev);
+    ret = vhost_dev_start(&fs->vhost_dev, vdev, true);
     if (ret < 0) {
         error_report("Error starting vhost: %d", -ret);
         goto err_guest_notifiers;
@@ -110,7 +110,7 @@ static void vuf_stop(VirtIODevice *vdev)
         return;
     }

-    vhost_dev_stop(&fs->vhost_dev, vdev);
+    vhost_dev_stop(&fs->vhost_dev, vdev, true);

     ret = k->set_guest_notifiers(qbus->parent, fs->vhost_dev.nvqs, false);
     if (ret < 0) {
diff --git a/hw/virtio/vhost-user-gpio.c b/hw/virtio/vhost-user-gpio.c
index 5851cb3bc9..0b40ebd15a 100644
--- a/hw/virtio/vhost-user-gpio.c
+++ b/hw/virtio/vhost-user-gpio.c
@@ -81,7 +81,7 @@ static int vu_gpio_start(VirtIODevice *vdev)
      */
     vhost_ack_features(&gpio->vhost_dev, feature_bits, vdev->guest_features);

-    ret = vhost_dev_start(&gpio->vhost_dev, vdev);
+    ret = vhost_dev_start(&gpio->vhost_dev, vdev, false);
     if (ret < 0) {
         error_report("Error starting vhost-user-gpio: %d", ret);
         goto err_guest_notifiers;
@@ -139,7 +139,7 @@ static void vu_gpio_stop(VirtIODevice *vdev)
         return;
     }

-    vhost_dev_stop(vhost_dev, vdev);
+    vhost_dev_stop(vhost_dev, vdev, false);

     ret = k->set_guest_notifiers(qbus->parent, vhost_dev->nvqs, false);
     if (ret < 0) {
diff --git a/hw/virtio/vhost-user-i2c.c b/hw/virtio/vhost-user-i2c.c
index 1c9f3d20dc..dc5c828ba6 100644
--- a/hw/virtio/vhost-user-i2c.c
+++ b/hw/virtio/vhost-user-i2c.c
@@ -46,7 +46,7 @@ static void vu_i2c_start(VirtIODevice *vdev)

     i2c->vhost_dev.acked_features = vdev->guest_features;

-    ret = vhost_dev_start(&i2c->vhost_dev, vdev);
+    ret = vhost_dev_start(&i2c->vhost_dev, vdev, true);
     if (ret < 0) {
         error_report("Error starting vhost-user-i2c: %d", -ret);
         goto err_guest_notifiers;
@@ -80,7 +80,7 @@ static void vu_i2c_stop(VirtIODevice *vdev)
         return;
     }

-    vhost_dev_stop(&i2c->vhost_dev, vdev);
+    vhost_dev_stop(&i2c->vhost_dev, vdev, true);

     ret = k->set_guest_notifiers(qbus->parent, i2c->vhost_dev.nvqs, false);
     if (ret < 0) {
diff --git a/hw/virtio/vhost-user-rng.c b/hw/virtio/vhost-user-rng.c
index f9084cde58..201a39e220 100644
--- a/hw/virtio/vhost-user-rng.c
+++ b/hw/virtio/vhost-user-rng.c
@@ -47,7 +47,7 @@ static void vu_rng_start(VirtIODevice *vdev)
     }

     rng->vhost_dev.acked_features = vdev->guest_features;
-    ret = vhost_dev_start(&rng->vhost_dev, vdev);
+    ret = vhost_dev_start(&rng->vhost_dev, vdev, true);
     if (ret < 0) {
         error_report("Error starting vhost-user-rng: %d", -ret);
         goto err_guest_notifiers;
@@ -81,7 +81,7 @@ static void vu_rng_stop(VirtIODevice *vdev)
         return;
     }

-    vhost_dev_stop(&rng->vhost_dev, vdev);
+    vhost_dev_stop(&rng->vhost_dev, vdev, true);

     ret = k->set_guest_notifiers(qbus->parent, rng->vhost_dev.nvqs, false);
     if (ret < 0) {
diff --git a/hw/virtio/vhost-vsock-common.c b/hw/virtio/vhost-vsock-common.c
index a67a275de2..d21c72b401 100644
--- a/hw/virtio/vhost-vsock-common.c
+++ b/hw/virtio/vhost-vsock-common.c
@@ -70,7 +70,7 @@ int vhost_vsock_common_start(VirtIODevice *vdev)
     }

     vvc->vhost_dev.acked_features = vdev->guest_features;
-    ret = vhost_dev_start(&vvc->vhost_dev, vdev);
+    ret = vhost_dev_start(&vvc->vhost_dev, vdev, true);
     if (ret < 0) {
         error_report("Error starting vhost: %d", -ret);
         goto err_guest_notifiers;
@@ -105,7 +105,7 @@ void vhost_vsock_common_stop(VirtIODevice *vdev)
         return;
     }

-    vhost_dev_stop(&vvc->vhost_dev, vdev);
+    vhost_dev_stop(&vvc->vhost_dev, vdev, true);

     ret = k->set_guest_notifiers(qbus->parent, vvc->vhost_dev.nvqs, false);
     if (ret < 0) {
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index d1c4c20b8c..7fb008bc9e 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1777,15 +1777,36 @@ int vhost_dev_get_inflight(struct vhost_dev *dev, 
uint16_t queue_size,
     return 0;
 }

+static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable)

There is a similarly-named vhost_set_vring_enable(NetClientState *nc,
int enable) function which is actually part of vhost_net. Please rename
it to vhost_net_set_vring_enable(). 

Should I rename it in this patch?

It should probably call
vhost_dev_set_vring_enable().

Ehm, the idea of this patch was to touch as little as possible to avoid 
new regressions.

Also, the semantics of vhost_dev_set_vring_enable() was meant to keep 
vhost_dev_start()/vhost_dev_stop() simple, not to be exposed to 
frontends. (maybe I should have written it, sorry about that).

However I agree that we should clean up vhost-net and also the other 
frontends as Raphael also suggested, but honestly I'm scared to do that 
now in this patch...

What I would have wanted to do, would be similar to what we do for 
vhost-vdpa: call SET_VRING_ENABLE in the vhost_ops->vhost_dev_start() 
callback of vhost-user.c.
Removing all the call to vhost_ops->vhost_set_vring_enable() in the 
frontends, but I think it's too risky to do that now.


+{
+    if (!hdev->vhost_ops->vhost_set_vring_enable) {
+        return 0;
+    }
+
+    /*
+     * For vhost-user devices, if VHOST_USER_F_PROTOCOL_FEATURES has not
+     * been negotiated, the rings start directly in the enabled state, and
+     * .vhost_set_vring_enable callback will fail since
+     * VHOST_USER_SET_VRING_ENABLE is not supported.
+     */
+    if (hdev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER &&
+        !virtio_has_feature(hdev->backend_features,
+                            VHOST_USER_F_PROTOCOL_FEATURES)) {
+        return 0;
+    }

These semantics are the opposite of vhost_user_set_vring_enable():

 if (!virtio_has_feature(dev->features, VHOST_USER_F_PROTOCOL_FEATURES)) {
     return -EINVAL;
 }

Please make vhost_user_set_vring_enable() and
vhost_dev_set_vring_enable() consistent. Code gets really confusing when
layers have different semantics for the same operation.

It's the opposite precisely because we shouldn't let 
vhost_dev_start()/vhost_dev_stop() fail if 
vhost_ops->vhost_set_vring_enable() can't be called because it would 
fail.

If I do it this way, then I have to put the check inside 
vhost_dev_start()/vhost_dev_stop(), and at this point I remove the 
function that would be useless (just a wrapper of 
hdev->vhost_ops->vhost_set_vring_enable).
Actually this was the first implementation I did, then I added the 
function just to have vhost_dev_start()/vhost_dev_stop() cleaner and to 
avoid duplicating the check.


+
+    return hdev->vhost_ops->vhost_set_vring_enable(hdev, enable);
+}

The return value is hard to understand. An error return is only returned
by vhost-user devices with VHOST_USER_F_PROTOCOL_FEATURES. There are
other cases that seem like they should return an error but return
success instead. For example, when called with enable=false on a
non-VHOST_USER_F_PROTOCOL_FEATURES device (e.g. vhost-kernel or legacy
vhost-user) we return success even though the vring wasn't disabled.

As I explained above, the idea was not to expose this function outside, 
but to use it only in vhost_dev_start()/vhost_dev_stop(). So the return 
value is 0 both when it has successes and when there is no need/way to 
enable/disable the vrings.

Perhaps since it is confusing, I will remove the function and put the 
code directly into vhost_dev_start()/vhost_dev_stop().

What do you think?

Thanks,
Stefano