Re: vhost-user-blk reconnect issue

[Michael S. Tsirkin]

On Mon, Apr 01, 2024 at 10:08:10AM +0800, Yajun Wu wrote:
> 
> On 3/27/2024 6:47 PM, Stefano Garzarella wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > 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.

CC Alex who wrote that commit.

> 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
> > >