From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60102) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fIzAq-000292-Ob for qemu-devel@nongnu.org; Wed, 16 May 2018 12:21:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fIzAn-0003vV-Kl for qemu-devel@nongnu.org; Wed, 16 May 2018 12:21:08 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51158 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fIzAn-0003vQ-Fg for qemu-devel@nongnu.org; Wed, 16 May 2018 12:21:05 -0400 Date: Wed, 16 May 2018 19:21:04 +0300 From: "Michael S. Tsirkin" Message-ID: <20180516183513-mutt-send-email-mst@kernel.org> References: <02887fa6-d7c8-bc39-6e65-3c3caec5b903@redhat.com> <20180419195610.GA29865@localhost.localdomain> <98f48c44-e55a-2bd2-0d41-5643f35bce68@redhat.com> <20180419215320.GC29865@localhost.localdomain> <20180512010007-mutt-send-email-mst@kernel.org> <20180516172121-mutt-send-email-mst@kernel.org> <2871c099-79fe-8432-12c9-637e743cc89c@redhat.com> <20180516180847-mutt-send-email-mst@kernel.org> <20180516153350.GM25013@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180516153350.GM25013@localhost.localdomain> Subject: Re: [Qemu-devel] [PATCH RESEND v2] i386/kvm: add support for KVM_CAP_X86_DISABLE_EXITS List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Paolo Bonzini , Wanpeng Li , qemu-devel@nongnu.org, kvm@vger.kernel.org, Radim =?utf-8?B?S3LEjW3DocWZ?= On Wed, May 16, 2018 at 12:33:50PM -0300, Eduardo Habkost wrote: > On Wed, May 16, 2018 at 06:13:17PM +0300, Michael S. Tsirkin wrote: > > On Wed, May 16, 2018 at 05:04:41PM +0200, Paolo Bonzini wrote: > > > On 16/05/2018 16:22, Michael S. Tsirkin wrote: > > > >> kvm-hint-dedicated=on only sets the CPUID bit, which Linux for example > > > >> uses that to disable pv spinlocks. "-realtime dedicated-cpus=on" only > > > >> disables the vmexits. You can use the two independently. > > > > > > > > But when would you want to use the two independently? > > > > > > 1) For testing > > > > > > 2) When some of your QEMUs are too old to support kvm-hint-dedicated=on, > > > you may still want to use -realtime dedicated-cpus=on to get better > > > performance on the new one. > > > > > > Paolo > > > > For the second purpose, can't we handle this using machine types? > > Machine-type compatibility code deals with defaults when options > are omitted, not for making the meaning of explicit options > depend on the machine-type. > > e.g. having "-machine pc-q35-2.11 -cpu ...,+kvm-hint-dedicated=on" > not expose the CPUID bit that was explicitly requested in the > command-line would be a bad idea. Why? We have machine type affecting guest visible device behaviours for years. > -- > Eduardo