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

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Message-ID: <537DBB12.6070108@redhat.com>
Date:	Thu, 22 May 2014 10:53:38 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Michael Mueller <mimu@...ux.vnet.ibm.com>,
	Alexander Graf <agraf@...e.de>
CC:	linux-s390@...r.kernel.org, kvm@...r.kernel.org,
	Gleb Natapov <gleb@...nel.org>, qemu-devel@...gnu.org,
	linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"Jason J. Herne" <jjherne@...ux.vnet.ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Andreas Faerber <afaerber@...e.de>,
	Richard Henderson <rth@...ddle.net>,
	Eduardo Habkost <ehabkost@...hat.com>
Subject: Re: [Qemu-devel] [PATCH v1 RFC 6/6] KVM: s390: add cpu model support

Il 22/05/2014 10:23, Michael Mueller ha scritto:
> On Wed, 21 May 2014 15:22:35 +0200
> Alexander Graf <agraf@...e.de> 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?

Paolo
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