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Re: [Qemu-devel] [PATCH v1 RFC 6/6] KVM: s390: add cpu model support

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH v1 RFC 6/6] KVM: s390: add cpu model support
Date: Fri, 23 May 2014 00:39:07 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 22.05.14 22:36, Christian Borntraeger wrote:
On 22/05/14 10:53, Paolo Bonzini wrote:
Il 22/05/2014 10:23, Michael Mueller ha scritto:
On Wed, 21 May 2014 15:22:35 +0200
Alexander Graf <address@hidden> wrote:

I have seen the slides from Eduardo which he presented during this years
DevConf in Brno and made my comments according the s390x implementation
on that. Is you will see, this is mostly overlapping except for the model
definition authority that I clearly see on qemu's side.

See pdf attachment.
More comments:

- "Only one machine type in s390 case which is -machine s390-virtio-ccw"

This probably should change sooner or later, as soon as the implementation 
becomes stable enough.  Versioning is necessary for live migration across 
different QEMU version.  Perhaps start versioning in 2.2, i.e. start making 
s390-virtio-ccw-2.1 an alias for s390-virtio-ccw now?

Note that new virtio device features can appear at any time outside the s390 
code, and will take part in versioning as well.

- "No enforce option"

Strongly suggest making enforce the only possible behavior.

- "Not in the s390x case, because the KVM facility mask limits the cpu model 
specific facilities"

What if the KVM facility mask changes?  For x86, nowadays new CPUID bits are 
only introduced in KVM when a new processors comes out.  But if we introduced 
an older CPUID bit, it would be a huge complication for backwards 
compatibility.  Is it different for s390?


I guess we need to have a full picture here. Would this topic be suitable for 
the KVM call?

Very much so, yes. Please put it on the agenda.


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