On 21/06/2016 11:21, Kevin Wolf wrote: > This series converts all I/O function in the core block layer up to > bdrv_co_preadv/pwritev() to taking a BdrvChild as their first parameter > instead of a BlockDriverState. > > The original motivation for this change were op blockers, where one of > the biggest problems is making sure that every user of block devices > actually registers correctly with the op blockers system. If the I/O > functions know which parent a request comes from (BdrvChild basically > corresponds to an edge in our block device graph), it can use assertions > to make sure that that parent has actually registered its activities and > thereby ensured that it doesn't conflict with other users. > > There are, however, more benefits we get from this change. The most > important one is probably that it enforces important aspects of the > block layer design like that external users go through a BlockBackend > and request are internally routed along the edges of the graph. Accesses > to random BDSes are no longer possible, you need to own an actual child > reference so you can make a request.
I still fail to understand what is the rationale for this change. The API is weird; you read from a disk, not from an edge, and in fact the first thing all the APIs do is dereference the BdrvChild... The assertions are nice, but I'm not sure it's a good idea to design a whole API around them. Paolo > The work on this series already led to a few cleanups and BlockBackend > conversions in master, and this series contains a few more. > > As a bonus, all the block drivers using bs->file->bs everywhere can now > go back to bs->file, which is a little nicer to read.
