From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43095) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bFJuX-0003iV-Lf for qemu-devel@nongnu.org; Tue, 21 Jun 2016 07:32:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bFJuV-00057P-Jm for qemu-devel@nongnu.org; Tue, 21 Jun 2016 07:32:04 -0400 Date: Tue, 21 Jun 2016 13:31:46 +0200 From: Kevin Wolf Message-ID: <20160621113146.GE4520@noname.redhat.com> References: <1466500894-9710-1-git-send-email-kwolf@redhat.com> <3f872c08-06d8-d755-9369-02ecd0d6d000@redhat.com> <20160621105620.GD4520@noname.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH 00/17] block: Convert common I/O path to BdrvChild List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-block@nongnu.org, famz@redhat.com, jcody@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com 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