From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1NmpnU-0007cu-HR for qemu-devel@nongnu.org; Wed, 03 Mar 2010 09:43:36 -0500 Received: from [199.232.76.173] (port=33073 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NmpnU-0007cV-3F for qemu-devel@nongnu.org; Wed, 03 Mar 2010 09:43:36 -0500 Received: from Debian-exim by monty-python.gnu.org with spam-scanned (Exim 4.60) (envelope-from ) id 1NmpnT-0005gP-CH for qemu-devel@nongnu.org; Wed, 03 Mar 2010 09:43:35 -0500 Received: from mx20.gnu.org ([199.232.41.8]:51084) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1NmpnT-0005gL-54 for qemu-devel@nongnu.org; Wed, 03 Mar 2010 09:43:35 -0500 Received: from mail.codesourcery.com ([38.113.113.100]) by mx20.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1NmpnR-0002ub-LI for qemu-devel@nongnu.org; Wed, 03 Mar 2010 09:43:33 -0500 From: Paul Brook Subject: Re: [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options Date: Wed, 3 Mar 2010 14:43:29 +0000 References: <201003022241.27803.paul@codesourcery.com> <4B8E6EF3.5020605@codemonkey.ws> In-Reply-To: <4B8E6EF3.5020605@codemonkey.ws> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201003031443.30218.paul@codesourcery.com> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: quintela@redhat.com, Marcelo Tosatti , qemu-devel@nongnu.org, "Michael S. Tsirkin" , kraxel@redhat.com, amit.shah@redhat.com > > That sounds like it's likely to come back and bite you. The guest has no > > idea which areas of ram happen to be contiguous on the host. > > Practically speaking, with target-i386 anything that is contiguous in > guest physical memory is contiguous in the host address space provided > it's ram. > > These assumptions are important. I have a local branch (that I'll send > out soon) that implements a ram API and converted virtio to make use of > it. I'm seeing a ~50% increase in tx throughput. IMO supporting discontiguous regions is a requirement. target-i386 might get away with contiguous memory, because it omits most of the board level details. For everything else I'd expect this to be a real problem. I'm not happy about the not-remapable assumption either. In my experience this is fairly common. In fact many real x86 machines have this capability (to workaround the 4G PCI hole). By my reading the ppc440_bamboo board fails both your assumptions. I imagine the KVM-PPC folks would be upset if you decided that virtio no longer worked on this board. This is all somewhat disappointing, given virtio is supposed to be a DMA based architecture, and not rely on shared memory semantics. Paul