From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MDeDV-00035h-CG for qemu-devel@nongnu.org; Mon, 08 Jun 2009 08:44:45 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MDeDQ-00033v-QN for qemu-devel@nongnu.org; Mon, 08 Jun 2009 08:44:45 -0400 Received: from [199.232.76.173] (port=52186 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MDeDQ-00033q-NJ for qemu-devel@nongnu.org; Mon, 08 Jun 2009 08:44:40 -0400 Received: from gecko.sbs.de ([194.138.37.40]:24819) by monty-python.gnu.org with esmtps (TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1MDeDQ-000544-0L for qemu-devel@nongnu.org; Mon, 08 Jun 2009 08:44:40 -0400 Message-ID: <4A2D07A3.8090101@siemens.com> Date: Mon, 08 Jun 2009 14:44:19 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <4A26F1E3.1040509@codemonkey.ws> <4A27FC69.9070501@mayc.ru> <20090605201415.GA22847@csclub.uwaterloo.ca> <20090608001312.GE15426@shareable.org> <4A2CA8C2.2080004@redhat.com> <20090608115755.GD25684@shareable.org> <4A2CFE07.90700@redhat.com> <20090608121626.GF25684@shareable.org> <4A2D03E7.8070702@redhat.com> In-Reply-To: <4A2D03E7.8070702@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: POLL: Why do you use kqemu? List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Anton D Kachalov , "qemu-devel@nongnu.org" , Lennart Sorensen Avi Kivity wrote: > Jamie Lokier wrote: >> Is there a reason to duplicate QEMU's task switch emulation, instead >> of trapping out to QEMU? Modern OSes don't use x86 task switching >> (because it's slow on real CPUs) except for ring stack switches, so >> it's hardly a performance requirement. Accurate task switch support >> is fiddly to get right. Think of all the exceptions including >> paging/segment exceptions in the middle of reading the TSS block. >> > > kvm is designed to be useful without full emulation in userspace. > And the fact that kqemu has to use tcg in order to achieve a reasonable performance is rather a disadvantage. The complexity and overhead for synchronizing tcg with the in-kernel accelerator is enormous. If there were a feasible way to overcome this with kqemu, it would benefit a lot. But unfortunately there is none (given you don't want to invest reasonable efforts). Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux