RE: [PATCH v2 1/4] hw/virtio: incorporate backend features in features

[Yajun Wu]

Hi Alex,

With this change, VHOST_USER_F_PROTOCOL_FEATURES bit will be set to backend for 
virtio block device (previously not).

From https://www.qemu.org/docs/master/interop/vhost-user.html spec:
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.

Vhost-user-blk won't send out VHOST_USER_SET_VRING_ENABLE today. 
Backend gets VHOST_USER_F_PROTOCOL_FEATURES negotiated and can't get 
VHOST_USER_SET_VRING_ENABLE.
VQs keep in disabled state.

Can you check on this scenario?

Thanks

-----Original Message-----
From: Qemu-devel <qemu-devel-bounces+yajunw=nvidia.com@nongnu.org> On Behalf Of 
Alex Bennée
Sent: Thursday, July 28, 2022 9:55 PM
To: qemu-devel@nongnu.org
Cc: mst@redhat.com; Alex Bennée <alex.bennee@linaro.org>
Subject: [PATCH v2 1/4] hw/virtio: incorporate backend features in features

External email: Use caution opening links or attachments


There are some extra bits used over a vhost-user connection which are hidden 
from the device itself. We need to set them here to ensure we enable things 
like the protocol extensions.

Currently net/vhost-user.c has it's own inscrutable way of persisting this data 
but it really should live in the core vhost_user code.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220726192150.2435175-7-alex.bennee@linaro.org>
---
 hw/virtio/vhost-user.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c index 
75b8df21a4..1936a44e82 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -1460,7 +1460,14 @@ static int vhost_user_set_features(struct vhost_dev *dev,
      */
     bool log_enabled = features & (0x1ULL << VHOST_F_LOG_ALL);

-    return vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES, features,
+    /*
+     * We need to include any extra backend only feature bits that
+     * might be needed by our device. Currently this includes the
+     * VHOST_USER_F_PROTOCOL_FEATURES bit for enabling protocol
+     * features.
+     */
+    return vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES,
+                              features | dev->backend_features,
                               log_enabled);  }

--
2.30.2