From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:35096) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bFJR8-0002lB-71 for qemu-devel@nongnu.org; Tue, 21 Jun 2016 07:01:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bFJR7-0005nx-39 for qemu-devel@nongnu.org; Tue, 21 Jun 2016 07:01:42 -0400 References: <1466500894-9710-1-git-send-email-kwolf@redhat.com> <3f872c08-06d8-d755-9369-02ecd0d6d000@redhat.com> <20160621105620.GD4520@noname.redhat.com> From: Paolo Bonzini Message-ID: Date: Tue, 21 Jun 2016 13:01:30 +0200 MIME-Version: 1.0 In-Reply-To: <20160621105620.GD4520@noname.redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit 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: Kevin Wolf Cc: qemu-block@nongnu.org, famz@redhat.com, jcody@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com On 21/06/2016 12:56, Kevin Wolf wrote: > Am 21.06.2016 um 11:47 hat Paolo Bonzini geschrieben: >> >> >> 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. > > 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. Paolo