Stefan Hajnoczi <[email protected]> writes:
> [[PGP Signed Part:Undecided]] > 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. I had to patch out that config_ops check to get the get_config over vhost to work. Otherwise QEMU keeps complaining: qemu-system-aarch64: VHOST_USER_PROTOCOL_F_CONFIG not supported because it itself has squashed the feature in the vhost protocol negotiation. > > Stefan > > [[End of PGP Signed Part]] -- Alex Bennée
