Re: [RFC PATCH 0/4] block layer: split block APIs in graph and I/O

[Paolo Bonzini]

On 13/09/21 15:10, Stefan Hajnoczi wrote:
> On Wed, Sep 08, 2021 at 09:10:17AM -0400, Emanuele Giuseppe Esposito wrote:
> > Currently, block layer APIs like block-backend.h contain a mix of
> > functions that are either running in the main loop and under the
> > BQL, or are thread-safe functions and run in iothreads performing I/O.
> > The functions running under BQL also take care of modifying the
> > block graph, by using drain and/or aio_context_acquire/release.
> > This makes it very confusing to understand where each function
> > runs, and what assumptions it provided with regards to thread
> > safety.
> >
> > We call the functions running under BQL "graph API", and
> > distinguish them from the thread-safe "I/O API".
>
> Maybe "BQL" is clearer than "graph" because not all functions classified
> as "graph" need to traverse/modify the graph.

Bikeshedding, I like it! :)

... on the other hand, qemu-storage-daemon does not have a BQL (see 
patch 1); "graph API" functions run from the main (monitor) thread.

The characteristic of the "graph API" is that they affect global state, 
so another possibility could be "global state API".  But is there any 
global state apart from the BlockDriverState graph and the associated 
BlockBackends?

Paolo