Re: [PATCH] qemu: Add vhost-user-blk unix socket to support server mode

On Thu, Mar 09, 2023 at 16:58:07 +0800, Zhenguo Yao wrote: > qemu support server mode when using vhost-user-blk disk. > Let libvirt to support this. Could you please elaborate how you expect to use this? I'm asking because server mode comes with a integrated set of race conditions:

> diff --git a/tests/qemuxml2argvdata/disk-vhostuser-numa.x86_64-latest.args b/tests/qemuxml2argvdata/disk-vhostuser-numa.x86_64-latest.args > index 75b3232dad..ea4b227328 100644 > --- a/tests/qemuxml2argvdata/disk-vhostuser-numa.x86_64-latest.args > +++ b/tests/qemuxml2argvdata/disk-vhostuser-numa.x86_64-latest.args > @@ -31,9 +31,11 @@ XDG_CONFIG_HOME=/tmp/lib/domain--1-QEMUGuest1/.config \ > -device '{"driver":"piix3-usb-uhci","id":"usb","bus":"pci.0","addr":"0x1.0x2"}' \ > -chardev socket,id=chr-vu-virtio-disk0,path=/tmp/vhost1.sock \ > -device '{"driver":"vhost-user-blk-pci","bus":"pci.0","addr":"0x2","chardev":"chr-vu-virtio-disk0","id":"virtio-disk0","bootindex":1}' \ > --chardev socket,id=chr-vu-virtio-disk1,path=/tmp/vhost1.sock,reconnect=10 \ > +-chardev socket,id=chr-vu-virtio-disk1,path=/tmp/vhost1.sock,server=on,wait=off \ Emphasizing that 'wait=off' is used for the chardev backend. When starting such a VM assuming only 1 storage device is being started you have a very short window where the client must connect to it and start the vhost session. Otherwise the disk will not be visible by the VM and it will not boot. This is a bit different than vhost-user network as it's required right at the start. Even then vhost-user networking which supports server mode sets 'wait=on'. But this is problematic on a different level. The VM is stuck until the vhost connection is established Similarly it also creates a problem when hotpluggingg as there 'wait=on' doesn't really exist, thus it behaves differently.

Thus unless there is a good reason to do so I find that the 'server' mode is not useful here.