From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1JtmTg-0001t5-5E for qemu-devel@nongnu.org; Wed, 07 May 2008 12:26:48 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1JtmTe-0001sY-NU for qemu-devel@nongnu.org; Wed, 07 May 2008 12:26:47 -0400 Received: from [199.232.76.173] (port=41334 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1JtmTe-0001sF-JK for qemu-devel@nongnu.org; Wed, 07 May 2008 12:26:46 -0400 Received: from mail2.shareable.org ([80.68.89.115]) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1JtmTe-00020p-9A for qemu-devel@nongnu.org; Wed, 07 May 2008 12:26:46 -0400 Date: Wed, 7 May 2008 17:26:42 +0100 From: Jamie Lokier Subject: Re: [Qemu-devel] [4367] Align file accesses with cache=off (Kevin Wolf, Laurent Vivier) Message-ID: <20080507162642.GA7324@shareable.org> References: <4820D905.4020407@bellard.org> <48216579.3060204@suse.de> <20080507123733.GA2822@shareable.org> <4821A8F0.9070506@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4821A8F0.9070506@suse.de> Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: qemu-devel@nongnu.org Kevin Wolf wrote: > No, nobody mentioned the recursion problem. Hmm. I concede you're right in the sense that it was mentioned, but on a different thread about QEMU AIO recently :-) Message-ID: <20080403113128.GA17900@shareable.org> Avi Kivity wrote: > Long term we want to replace the recursion by queuing. Ah yes. Reminds me of a bug in a program of mine with asynchronous sockets. I learned a few lessons about async callbacks: > And you were talking about > problem with two different file descriptors for one file, not about the > fcntl solution. Ok, might also be that the hints were just not explicit > enough for me. ;-) Following the paragraph about two file descriptors, there was: >> I'm not sure if that works, though. On some OSes, if a file has any >> non-O_DIRECT open descriptor, all I/O is buffered ignoring the >> O_DIRECT flag. If both are allowed simultaneously, I'm not sure what >> happens with cache-coherency between direct I/Os and buffered I/Os. Not sure if that is quite the same thing :-) I did miss that switching O_DIRECT on/off while AIOs are in flight on that descriptor might be dodgy (implementation dependent), and that it might not do the right things w.r.t. cohrency. > But even if so, this is more of a general feeling about how patches are > handled and not only related to this patch. I agree and have a similar feeling, though it's not a bad thing provided the issues are actually noticed, which they do seem to be. I have the impression there are many people working on different specific features and subsystems, but not so much on overall architecture in a coordinated and "visionary" way. Most of my issues with QEMU are the epic list of difficulties with Microsoft guests (espcially when people send me images for a different VM), the peculiar divergence between KVM and QEMU features, and the awkwardness of the monitor/command line interface. Since I'm not an active code contributor, and those are to a great extent feature requests or only debuggable by users, I keep those thoughts largely to myself :-) -- Jamie