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Re: [Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt
Date: Mon, 12 Mar 2012 15:30:36 -0300
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, Mar 12, 2012 at 06:41:06PM +0100, Andreas Färber wrote:
> Am 12.03.2012 17:50, schrieb Eduardo Habkost:
> > On Mon, Mar 12, 2012 at 04:49:47PM +0100, Andreas Färber wrote:
> >> IMO interpreting an explicit -cpu parameter depending on -M would be
> >> wrong. Changing the default CPU based on -M is fine with me. For an
> >> explicit argument we would need Westmere-1.0 analog to pc-1.0. Then the
> >> user gets what the user asks for, without unexpected magic.
> > 
> > It is not unexpected magic. It would be a documented mechanism:
> > "-cpu Nehalem-1.0" and "-cpu Nehalem-1.1" would have the same meaning
> > every time, with any machine-type, but "-cpu Nehalem" would be an alias,
> > whose meaning depends on the machine-type.
> > 
> > Otherwise we would be stuck with a broken "Nehalem" model forever, and
> > we don't want that.
> Not quite what I meant: In light of QOM we should be able to instantiate
> a CPU based on its name and optional parameters IMO. No dependency on
> the machine, please. An alias sure, but if the user explicitly says -cpu
> Nehalem then on 1.1 it should always be an alias to Nehalem-1.1 whether
> the machine is -M pc-0.15 or pc. If no -cpu was specified by the user,
> then choosing a default of Nehalem-1.0 for pc-1.0 is fine. Just trying
> to keep separate things separate here.

As Gleb explained, things aren't really separated:
"qemu-1.1 -M pc-1.0 -cpu Nehalem" should result in the same machine as
"qemu-1.0 -cpu Nehalem", no difference should be visible to the guest.
simply make incompatible changes.

> Also keep in mind linux-user. There's no concept of a machine there, but
> there's a cpu_copy() function used for forking that tries to re-create
> the CPU based on its model. So currently cpu_*_init(env->cpu_model_str)
> needs to be able to recreate an identical CPU through the central code
> path, without access to a QEMUMachine.

So just translate the CPU alias given to "-cpu" to the true CPU model
name as soon as possible, at the command-line-handling code, so the rest
of the code always see the true CPU model name.

After all, the need to make the aliases is a command-line interface
compatibility problem, so it makes sense to handle this at the
command-line-handling code.


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