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Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-versi

From: Andrea Bolognani
Subject: Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max"
Date: Tue, 10 Apr 2018 09:41:33 +0200

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.

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 ;)

Andrea Bolognani / Red Hat / Virtualization

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