Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type "device IO"

[Paolo Bonzini]

On 28/05/2015 12:55, Fam Zheng wrote:
> > Indeed. blk_pause/resume would handle everything in one central place
> > in the block layer instead of spreading the logic across all the block
> > layer users.
>
> Sorry, I'm confused. Do you mean there is a way to implement blk_pause
> completely in block layer, without the necessity of various notifier handlers
> in device models?

How would you do that?  Do you have to keep a queue of pending requests
in the BlockBackend?  Since bdrv_drain_all may never return (e.g. stuck
NFS connection with nfs=hard), the guest can force unbounded allocation
in the host, which is bad.

In addition, the BDS doesn't have a list of BlockBackends attached to
it.  So you need the BlockBackends to register themselves for
pause/resume in some way---for example with a notifier list.

Then it's irrelevant whether it's the device model or the BB that
attaches itself to the notifier list.  You can start with doing it in
the device models (those that use ioeventfd), and later it can be moved
to the BB.  The low-level implementation remains the same.

Paolo