On Fri, Mar 04, 2022 at 04:49:30PM +0000, Alex Bennée wrote: > > Stefan Hajnoczi <[email protected]> writes: > > > [[PGP Signed Part:Undecided]] > > On Mon, Feb 28, 2022 at 04:16:43PM +0000, Alex Bennée wrote: > >> > >> Stefan Hajnoczi <[email protected]> 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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