Qemu block filter insertion/removal API

[Vladimir Sementsov-Ogievskiy]

Hi all!

I'd like to be sure that we know where we are going to.

In blockdev-era where qemu user is aware about block nodes, all nodes have good 
names and controlled by user we can efficiently use block filters.

We already have some useful filters: copy-on-read, throttling, compress. In my 
parallel series I make backup-top filter public and useful without backup block 
jobs. But now filters could be inserted only together with opening their child. 
We can specify filters in qemu cmdline, or filter can take place in the block 
node chain created by blockdev-add.

Still, it would be good to insert/remove filters on demand.

Currently we are going to use x-blockdev-reopen for this. Still it can't be used to 
insert a filter above root node (as x-blockdev-reopen can change only block node options 
and their children). In my series "[PATCH 00/21] block: publish backup-top 
filter" I propose (as Kevin suggested) to modify qom-set, so that it can set drive 
option of running device. That's not difficult, but it means that we have different 
scenario of inserting/removing filters:

1. filter above root node X:

inserting:

  - do blockdev-add to add a filter (and specify X as its child)
  - do qom-set to set new filter as a rood node instead of X

removing

  - do qom-set to make X a root node again
  - do blockdev-del to drop a filter

2. filter between two block nodes P and X. (For example, X is a backing child 
of P)

inserting

  - do blockdev-add to add a filter (and specify X as its child)
  - do blockdev-reopen to set P.backing = filter

remvoing

  - do blockdev-reopen to set P.backing = X
  - do blockdev-del to drop a filter


And, probably we'll want transaction support for all these things.


Is it OK? Or do we need some kind of additional blockdev-replace command, that 
can replace one node by another, so in both cases we will do

inserting:
 
  - blockdev-add filter
  - blockdev-replace (make all parents of X to point to the new filter instead 
(except for the filter itself of course)

removing
  
  - blockdev-replace (make all parante of filter to be parents of X instead)
  - blockdev-del filter


It's simple to implement, and it seems for me that it is simpler to use. Any 
thoughts?

--
Best regards,
Vladimir