Re: [Qemu-arm] [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max"

["Daniel P. Berrangé" <berrange@redhat.com>]

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
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