Re: [Qemu-devel] virtio-9p: hot-plug/unplug about virtio-9p device

[Greg Kurz <groug@kaod.org>]

On Tue, 19 Dec 2017 13:41:12 +0800
sochin.jiang <sochin.jiang@huawei.com> wrote:

>      Hi, guys.
> 
>     I'm looking for the hot-plug/unplug features of virtio-9p device recently, and found there's a lack of support.
> 
>     I am wondering why ? Is there a reason. Actually, I write a qmp command to support fsdev_add, then a device_add qmp will    
> 
>     successfully add a virtio-9p device(just like virtio-blk).
> 
>    Whether there is some concerns that we haven't support this ? hope for a reply, thanks.
> 
> 
> sochin.
> 

Hi Sochin,

I've just discovered this mail by chance. Please note I'm the only active
maintainer for virtio-9p. You really should check the content of MAINTAINERS
and Cc the appropriate people. :)

Now, back to your question. Yes, there's only some partial support for
hot-plug/unplug. Mostly because nobody cared to work on it I guess.

So, indeed, we don't have fsdev_add/fsdev_del, ie, we can only rely on
shared directories specified on the QEMU command line.

On the virtio-9p device side, hotplug is supported, ie, device_add virtio-9p
works as expected.

Hot-unplug is different as it requires some coordination with the guest. The
current status is that it requires the 9p shared directory to be unmounted
in the guest: if you do device_del while the directory is mounted in a linux
guest, you'll get these messages in the guest syslog:

kernel:9pnet_virtio virtio2: p9_virtio_remove: waiting for device in use.

If the 9p directory is unmounted at some point, then the hot unplug
sequence will eventually succeed.

But this shouldn't be done like this: the guest should cancel inflight
requests and cause any new I/O requests in the guest to fail right away.
I have a tentative patch for the linux driver I can share if you want.

Cheers,

--
Greg