Re: [PATCH RFC 0/3] virtio: add new notify() callback to virtio_driver

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 20, 2013 at 04:22:00PM +0100, Heinz Graalfs wrote:
> Hi,
> 
> when an active virtio block device is hot-unplugged from a KVM guest, running
> affected guest user applications are not aware of any errors that occur due
> to the lost device. This patch-set adds code to avoid further request queueing
> when a lost block device is detected, resulting in appropriate error info.
> 
> On System z there exists no handshake mechanism between host and guest
> when a device is hot-unplugged. The device is removed and no further I/O
> is possible.
> 
> When an online channel device disappears on System z the kernel's CIO layer
> informs the driver (virtio_ccw) about the lost device.
> 
> It's just the block device drivers that care to provide a notify
> callback.
> 
> Here are some more error details:
> 
> For a particular block device virtio's request function virtblk_request()
> is called by the block layer to queue requests to be handled by the host.
> In case of a lost device requests can still be queued, but an appropriate
> subsequent host kick usually fails. This leads to situations where no error
> feedback is shown.
> 
> In order to prevent request queueing for lost devices appropriate settings
> in the block layer should be made. Exploiting System z's CIO notify handler
> callback, and adding a corresponding new virtio_driver notify() handler to
> 'inform' the block layer, solve this task.
> 
> Patch 1 adds an optional notify() callback to virtio_driver.
> 
> Patch 2 adds a new notify() callback for the virtio_blk driver. When called
> for a lost device settings are made to prevent future request queueing.
> 
> Patch 3 modifies the CIO notify handler in virtio_ccw's transport layer to pass
> on the lost device info to virtio's backend driver virtio_blk.

Question: I guess remove callback is invoked eventually?
Could you please clarify why isn't this sufficient?



> Heinz Graalfs (3):
>   virtio: add notify() callback to virtio_driver
>   virtio_blk: add virtblk_notify() as virtio_driver's notify() callback
>   virtio_ccw: invoke virtio_driver's notify() on CIO_GONE notification
> 
>  drivers/block/virtio_blk.c    | 14 ++++++++++++++
>  drivers/s390/kvm/virtio_ccw.c | 14 ++++++++++++--
>  drivers/virtio/virtio.c       |  8 ++++++++
>  include/linux/virtio.h        | 10 ++++++++++
>  4 files changed, 44 insertions(+), 2 deletions(-)
> 
> -- 
> 1.8.3.1