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 <cpu> 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 :|