Re: [Qemu-devel] [PATCH 00/17] block: Convert common I/O path to BdrvChild

[Kevin Wolf]

Am 21.06.2016 um 13:01 hat Paolo Bonzini geschrieben:
> On 21/06/2016 12:56, Kevin Wolf wrote:
> > Am 21.06.2016 um 11:47 hat Paolo Bonzini geschrieben:
> >> 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.
> > 
> > Do you see a problem with such an API, though? If there is no reason not
> > to have the advantages, as small as they may seem, why not take them?
> 
> I don't see a reason not to take them; I don't see any red flags, but
> there are some yellow flags (the kinda weird API) that I don't
> understand and I hope you can explain.
> 
> Thinking more about it, it's perfectly possible that this is just a
> combination of block/io.c's growth by accretion and the well-known fact
> "naming pseudo-OOP member functions in C sucks".
> 
> In other words, if you sell me this as "let's add some member functions
> to BdrvChild and use them", I can buy it.  Perhaps the only thing to do
> then is to rename functions and design a consistent naming.

Hm, I never thought about it this way, but I think it actually makes
sense.

As we want to represent a graph where both nodes and edges can have
attributes and methods, OOP-wise both of them are objects, namely BDS
and BdrvChild.

So we have some BDS A that has a Child B, and Child B in turn has a
BDS C. What we used to do is that A asks B for the node it points to
(C), and then directly calls a method of C. After the conversion, A
calls a method of B, which in turn forwards the request by calling a
method of C, which is much more straightforward and ideally even allows
the node that B points to to remain private (we're not quite there,
though).

Kevin