From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1JyrY9-0007dN-If for qemu-devel@nongnu.org; Wed, 21 May 2008 12:52:25 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1JyrY8-0007cM-Fr for qemu-devel@nongnu.org; Wed, 21 May 2008 12:52:24 -0400 Received: from [199.232.76.173] (port=40328 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1JyrY8-0007cI-36 for qemu-devel@nongnu.org; Wed, 21 May 2008 12:52:24 -0400 Received: from bzq-179-150-194.static.bezeqint.net ([212.179.150.194]:53819 helo=il.qumranet.com) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1JyrQH-00018L-K3 for qemu-devel@nongnu.org; Wed, 21 May 2008 12:44:18 -0400 Message-ID: <4834515E.6070807@qumranet.com> Date: Wed, 21 May 2008 19:44:14 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT) References: <200805210205.37432.paul@codesourcery.com> <4833778C.4030209@codemonkey.ws> <4833DC3F.8000604@suse.de> <20080521122629.GA14416@shareable.org> <48341783.3060204@qumranet.com> <20080521134154.GA15210@shareable.org> <483429EB.7070705@codemonkey.ws> <48342F05.2090603@qumranet.com> <48343106.4070801@codemonkey.ws> <48343844.1050107@qumranet.com> <20080521153454.GB20527@shareable.org> In-Reply-To: <20080521153454.GB20527@shareable.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Blue Swirl , Laurent Vivier , Paul Brook Jamie Lokier wrote: > Avi Kivity wrote: > >> Here's a summary of the use cases I saw so far: >> >> - casual use, no critical data: write back cache >> >> - backing file shared among many guests: read-only, cached >> >> - desktop system, but don't lose my data: O_SYNC >> (significant resources on the host) >> >> - dedicated virtualization engine: O_DIRECT >> (most host resources assigned to guests) >> > > Sounds alright, but on _my_ desktop system (a laptop), I would use O_DIRECT. > > There isn't enough RAM in my system to be happy duplicating data in > guests and hosts at the same time. VMs are quite demanding on RAM. > > Sure, if you're low on resources, and aren't rebooting often, that's the right thing to do. > However, if you find a way to map host cached pages into the guest > without copying - so sharing the RAM - that would be excellent. It > can be done in principle, by remapping pages to satisfy IDE/SCSI DMA > requests. I don't know if it would be fast enough. Perhaps it would > work better in KVM than QEMU. > Sounds like a memory management nightmare. With mmu notifiers (or plain qemu), though, it can be done. Have the backing file also contain an area for guest RAM. Use a nonlinear mapping to map this area as guest memory. If the guest issues a properly-aligned read, call remap_file_pages() for that page, and write-protect it. When you get a protection violation (as the guest writes to that memory), copy it to the RAM area and remap it again. I don't think remap_file_pages() supports different protections in a single VMA; that could kill the whole idea. -- error compiling committee.c: too many arguments to function