From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41707) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f5p0y-0007wm-Sj for qemu-devel@nongnu.org; Tue, 10 Apr 2018 04:52:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f5p0u-0002yz-UZ for qemu-devel@nongnu.org; Tue, 10 Apr 2018 04:52:32 -0400 Date: Tue, 10 Apr 2018 09:52:00 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20180410085200.GD5155@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <20180409154921.29906-1-wei@redhat.com> <20180409155634.GO18283@redhat.com> <6bb12858-3b4e-e92f-93ea-b8708c8be870@redhat.com> <1523346093.16671.23.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1523346093.16671.23.camel@redhat.com> Subject: Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max" List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Andrea Bolognani Cc: Wei Huang , qemu-devel@nongnu.org, peter.maydell@linaro.org, drjones@redhat.com, qemu-arm@nongnu.org On Tue, Apr 10, 2018 at 09:41:33AM +0200, Andrea Bolognani wrote: > On Mon, 2018-04-09 at 11:29 -0500, Wei Huang wrote: > > > > Running mach-virt machine types (i.e. "-M virt") on different systems can > > > > result in various misleading warnings if -cpu and/or gic-version not specified. > > > > For KVM, this can be solved mostly by using "host" type. But the "host" type > > > > doesn't work for TCG. Compared with "host", the "max" type not only supports > > > > auto detection under KVM mode, but also works with TCG. So this patch set > > > > "max" as the default types for both -cpu and gic-version. > > > > > > Hmm, generally we aim for the config provided by a machine type to be stable > > > across QEMU versions. > > > > I understand this principle. But in reality, under KVM mode, the default > > config most time doesn't work. If end users specify cpu type manually, > > it still doesn't work because the host CPU is vendor-specific design > > (e.g. "cortex-a57" doesn't work on QCOM's machine). So we end up with > > using "-cpu host" all the time. My argument for this patch is that "-cpu > > max" isn't worse than "-cpu host". > > I figure the people not explicitly specifying a CPU model on the > command line will probably also use '-M virt' instead of versioned > machine types, which means they will get a different guest behavior > after upgrading QEMU regardless. Libvirt uses versioned machine types and does not specify -cpu unless the user has added to their XML. IOW libvirt assumes the default CPU model is stable because that's what QEMU has promised in the past. > Defaulting to 'max' for '-cpu' and 'gic-version' makes it convenient > to quickly and concisely start a guest; if you care about guest ABI > at all, then you are already specifying everything explicitly on the > command line instead of relying on defaults - or using libvirt ;) Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|