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

[Alex Bennée]

Yajun Wu <yajunw@nvidia.com> writes:

> 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.

If the backend advertises protocol features but the stub doesn't support
it how does it get enabled?

The testing I did was mostly by hand with the gpio backend and using the
qtests. I Think we need to add some acceptance testing into avocado with
some real daemons because I don't think we have enough coverage with the
current qtest approach.

>
> 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);  }


-- 
Alex Bennée