From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:60008) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UPwdQ-0007GR-Bl for qemu-devel@nongnu.org; Wed, 10 Apr 2013 11:08:33 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UPwdL-00087U-QX for qemu-devel@nongnu.org; Wed, 10 Apr 2013 11:08:28 -0400 Received: from mail-qe0-f52.google.com ([209.85.128.52]:44675) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UPwdL-00087M-Mb for qemu-devel@nongnu.org; Wed, 10 Apr 2013 11:08:23 -0400 Received: by mail-qe0-f52.google.com with SMTP id jy17so295456qeb.25 for ; Wed, 10 Apr 2013 08:08:23 -0700 (PDT) From: Anthony Liguori In-Reply-To: References: <20130405093124.GA2351@dhcp-200-207.str.redhat.com> <1365552342-22840-1-git-send-email-josh.durgin@inktank.com> Date: Wed, 10 Apr 2013 10:08:20 -0500 Message-ID: <87ip3uctjv.fsf@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Subject: Re: [Qemu-devel] [PATCH v3 2/2] rbd: link and load librbd dynamically List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi , Josh Durgin Cc: Kevin Wolf , Paolo Bonzini , qemu-devel , Stefan Hajnoczi Stefan Hajnoczi writes: > On Wed, Apr 10, 2013 at 2:05 AM, Josh Durgin wrote: > NACK > > I think we're solving the problem at the wrong level. Writing our own > dynamic linker and adding boilerplate to juggle function pointers > every time we use a library dependency is ugly. > > There are two related problems here: > > 1. Packagers do not want to enable niche dependencies since users will > complain that the package is bloated and pulls in too much stuff. > > 2. QEMU linked against a newer library version fails to run on hosts > that have an older library. > > Problem #1 has several solutions: > > 1. Let packagers take care of it. For example, vim is often shipped > in several packages that have various amounts of dependencies > (vim-tiny, vim-gtk, etc). Packagers create the specialized packages > for specific groups of users to meet their demands without dragging in > too many dependencies. > > 2. Make QEMU modular - host devices should be shared libraries that > are loaded at runtime. There should be no stable API so that > development stays flexible and we discourage binary-only modules. > This lets packagers easily ship a qemu-rbd package, for example, that > drops in a .so file that QEMU can load at runtime. > > Problem #2 is already solved: > > The dynamic linker will refuse to load the program if there are > missing symbols. It's not possible to mix and match binaries across > environments while downgrading their library dependencies. With > effort, this could be doable but it's not an interesting use case that > many users care about - they get their binaries from a distro or build > them from source with correct dependencies. > > Maybe it's time to move block drivers and other components into > modules? This is really a build system issue more than anything else. There are no internal API changes needed. All that's needed is to something like (in module.h): /* This should not be used directly. Use block_init etc. instead. */ #ifdef CONFIG_MODULE #define module_init(function, type) \ const gchar *g_module_check_init(GModule *module) \ { \ register_module_init(function, type); \ return NULL; \ } #else #define module_init(function, type) \ static void __attribute__((constructor)) do_qemu_init_ ## function(void) { \ register_module_init(function, type); \ } #endif We then also need a way to load modules prior to calling init using the GModule interfaces. Easiest thing to do is just load all .so's in a single directory (/usr/lib/qemu/modules/*.so?) prior to calling any module init functions. What we need from the build system is the ability to build things either builtin or as modules. Paolo has a GSoC proposal to integrate kconfig. This would be a great approach to solving this problem. Doing it this way would let us build not only block drivers but also devices as modules. This would let us make QXL a module making it easier for distros to not have a hard dependence on libspice for the QEMU package. Regards, Anthony Liguori > > Stefan