Re: vhost-user-blk reconnect issue

[Yajun Wu]

On 3/27/2024 6:47 PM, Stefano Garzarella wrote:
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>
> Hi Yajun,
>
> On Mon, Mar 25, 2024 at 10:54:13AM +0000, Yajun Wu wrote:
> > Hi experts,
> >
> > With latest QEMU (8.2.90), we find two vhost-user-blk backend reconnect
> > failure scenarios:
> Do you know if has it ever worked and so it's a regression, or have we
> always had this problem?

I am afraid this commit: "71e076a07d (2022-12-01 02:30:13 -0500) 
hw/virtio: generalise CHR_EVENT_CLOSED handling"  caused both failures. 
Previous hash is good.

I suspect the "if (vhost->vdev)" in vhost_user_async_close_bh is the 
cause, previous code doesn't have this check?

> Thanks,
> Stefano
>
> > 1. Disconnect vhost-user-blk backend before guest driver probe vblk device, 
> > then reconnect backend after guest driver probe device. QEMU won't send out any 
> > vhost messages to restore backend.
> > This is because vhost->vdev is NULL before guest driver probe vblk device, so 
> > vhost_user_blk_disconnect won't be called, s->connected is still true. Next 
> > vhost_user_blk_connect will simply return without doing anything.
> >
> > 2. modprobe -r virtio-blk inside VM, then disconnect backend, then reconnect 
> > backend, then modprobe virtio-blk. QEMU won't send messages in vhost_dev_init.
> > This is because rmmod will let qemu call vhost_user_blk_stop, vhost->vdev also 
> > become NULL(in vhost_dev_stop), vhost_user_blk_disconnect won't be called. Again 
> > s->connected is still true, even chr connect is closed.
> >
> > I think even vhost->vdev is NULL, vhost_user_blk_disconnect should be called 
> > when chr connect close?
> > Hope we can have a fix soon.
> >
> >
> > Thanks,
> > Yajun