Re: What is the correct way to handle the VirtIO config space in vhost-user?

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Mar 04, 2022 at 04:49:30PM +0000, Alex Bennée wrote:
> 
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
> > [[PGP Signed Part:Undecided]]
> > On Mon, Feb 28, 2022 at 04:16:43PM +0000, Alex Bennée wrote:
> >> 
> >> Stefan Hajnoczi <stefanha@redhat.com> writes:
> >> 
> >> > [[PGP Signed Part:Undecided]]
> >> > On Fri, Feb 25, 2022 at 05:32:43PM +0000, Alex Bennée wrote:
> >> >> 
> >> >> [Apologies to CC list for repost due to fat fingering the mailing list address]
> >> >> 
> >> <snip>
> >> >> 
> >> >> (aside: this continues my QOM confusion about when things should be in a
> >> >> class or instance init, up until this point I hadn't needed it in my
> >> >> stub).
> >> >
> >> > Class init is a one-time per-class initializer function. It is mostly
> >> > used for setting up callbacks/overridden methods from the base class.
> >> >
> >> > Instance init is like an object constructor in object-oriented
> >> > programming.
> >> 
> >> I phrased my statement poorly. What I meant to say is I sometimes find
> >> QEMUs approach to using class over instance initialisation inconsistent.
> >> I think I understand the "policy" as use class init until there is a
> >> case where you can't (e.g. having individual control of each instance of
> >> a device).
> >> 
> >> > This is not a .get_config() method, it's a VIRTIO configuration change
> >> > notification handler. The vhost-user-blk device server ("slave") sends
> >> > this notification to notify the driver that configuration space contents
> >> > have been updated (e.g. the disk was resized).
> >> 
> >> So this should come in the initial vhost-user set of handshake messages
> >> if the VHOST_USER_PROTOCOL_F_CONFIG is negotiated between the master and
> >> slave? I guess without this protocol feature vhost-user can't support
> >> writeable config spaces?
> >
> > The VHOST_USER_PROTOCOL_F_CONFIG vhost-user protocol feature bit
> > enables:
> > 1. VHOST_USER_GET_CONFIG - reading configuration space
> > 2. VHOST_USER_SET_CONFIG - writing configuration space
> > 3. VHOST_USER_SLAVE_CONFIG_CHANGE_MSG - change notifications
> >
> > If the vhost-user server is supposed to participate in configuration
> > space accesses/notifications, then it needs to implement
> > VHOST_USER_PROTOCOL_F_CONFIG.
> >
> > QEMU's vhost-user-blk assumes the vhost-user server supports
> > VHOST_USER_PROTOCOL_F_CONFIG. It's an optional vhost-user protocol
> > feature but the virtio-blk device relies on configuration space
> > (otherwise QEMU's --device vhost-user-blk wouldn't know the capacity of
> > the disk). vhost_user_blk_realize_connect() sends VHOST_USER_GET_CONFIG
> > to fetch the configuration space contents when the device is
> > instantiated.
> >
> > Some vhost-user device types don't need VHOST_USER_PROTOCOL_F_CONFIG. In
> > that case QEMU's --device vhost-user-FOO implements .get/set_config()
> > itself. virtio-net is an example where this is the case.
> 
> I wonder when the last time this was tested was because since 1c3e5a2617
> (vhost-user: back SET/GET_CONFIG requests with a protocol feature) the
> check in vhost_user_backend_init is:
> 
>    if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
>        /* Don't acknowledge CONFIG feature if device doesn't support it */
>        dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
>    } else if (!(protocol_features &
>                (1ULL << VHOST_USER_PROTOCOL_F_CONFIG))) {
>        error_setg(errp, "Device expects VHOST_USER_PROTOCOL_F_CONFIG "
>                   "but backend does not support it.");
>        return -EINVAL;
>    }
> 
> which means I don't think it ever asks the vhost-user backend.

Can you describe what you have in mind? The issue isn't clear to me.

Stefan

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