Re: [QEMU HCI-8.0 PATCH 4/5] vhost-blk: watch the device for resize events

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

On 9/3/26 16:56, Andrey Drobyshev wrote:
> The approach introduced by this and subsequent patches, i.e. subscribing
> to NETLINK_KOBJECT_UEVENT and filtering them - that looks very kludgy
> for production and will unlikely be ever accepted upstream.

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
>