[Qemu-devel] RFC: block: combine read and write routines into rw_vector

[Michael Tokarev]

The following 3-patch series performs conversion of read and
write methods of block drivers into bdrv_rw_vector with
additional is_write argument.

Current I/O request path is twisted so much that it becomes
very difficult to follow.  A read or write request goes from
common io routine to individual read or write, back to common
on the next layer, again to individual read or write, and
so on.

The idea is to have common path as long as reads are not
differ from writes much (or at all), and split them into
individual routines when they actually become different.

So one result of this change is that in some drivers, we
now have tiny multiplexors which calls one or another
routine based on is_write argument.  These are mostly
old drivers, and also drivers which have to deal with
metadata.  There's no multiplexors all over the path
anymore.

A side effect of this change is removal of quite some
duplication, and combining several very similar code
paths together.

Probably the most interestig and questionable place is
the 3rd patch, in block.c, where I combined 3 routines
into one.  I like the result better than 3 separate
routines, especially because before this change, the
fill_zeroes path was too twisted.

While at it, the change fixes a bug - current block
layer can derererence NULL pointers at will -- in
case when only read bdrv function is provided but
no write (or the reverse), block layer does not check
if the pointer is NULL and calls it anyway.  I moved
the checks into individual block drivers.

Please note that this is just an RFC.  I run-tested
(it compiles on each step) only few drivers and only
ensured basic operation (it boots a windows guest and
performs some i/o).

My motivation initially was to verify my understanding
of the block layer, but once I hit the twisted nature
of it I thought I may fix this while at that, too.

Thanks,

/mjt