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I also think qmp command is much cleaner. > > So, QEMU must get the knowledge about updated size. It can only do so > by getting this knowledge from libvirt (as it used to be with > qmp_block_resize) or from the kernel via some kind of notification. Updated size can be read again with llseek/BLKSZGET. I would avoid passing it into QEMU explicitly via QMP (we also don't pass it during the start, so it matches) > > So our options: > > 1. Another QMP command sent directly from libvirt, smth like > 'vhost-blk-config-update id=virtio-disk0 data={...}' > > 2. Patching the kernel so it knows the size, caches it. Making ploop > send an ioctl to inform the kernel about new size. Then establishing > eventfd from vhost-blk backend -> qemu, and processing the events. > > I think 1st is cleaner and requires less work. > > Andrey >