Re: [Qemu-devel] [PATCH 2/2] KVM: Use -cpu best as default on x86

[Ryan Harper <ryanh@us.ibm.com>]

* Alexander Graf <agraf@suse.de> [2012-01-16 13:37]:
> 
> On 16.01.2012, at 20:30, Ryan Harper wrote:
> 
> > * Alexander Graf <agraf@suse.de> [2012-01-08 17:53]:
> >> When running QEMU without -cpu parameter, the user usually wants a sane
> >> default. So far, we're using the qemu64/qemu32 CPU type, which basically
> >> means "the maximum TCG can emulate".
> > 
> > it also means we all maximum possible migration targets.  Have you
> > given any thought to migration with -cpu best? 
> 
> If you have the same boxes in your cluster, migration just works. If
> you don't, you usually use a specific CPU model that is the least
> dominator between your boxes either way.

Sure, but the idea behind -cpu best is to not have to figure that out;
you had suggested that the qemu64/qemu32 were just related to TCG, and
what I'm suggesting is that it's also the most compatible w.r.t
migration.  

it sounds like if migration is a requirement, then -cpu best probably
isn't something that would be used.  I suppose I'm OK with that, or at
least I don't have a better suggestion on how to carefully push up the
capabilities without at some point breaking migration.

> 
> The current kvm64 type is broken. Libgmp just abort()s when we pass it
> in. So anything is better than what we do today on AMD hosts :).

I wonder if it breaks with Cyris cpus... other tools tend to do runtime
detection (mplayer).


> 
> 
> Alex

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com