linux-kernel - Re: [Qemu-devel] [PATCH v5 15/17] target-s390x: Extend arch specific QMP command query-cpu-definitions

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Date:	Mon, 27 Apr 2015 12:29:23 +0200
From:	Michael Mueller <mimu@...ux.vnet.ibm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
	Eduardo Habkost <ehabkost@...hat.com>,
	Gleb Natapov <gleb@...nel.org>, Alexander Graf <agraf@...e.de>,
	Daniel Hansel <daniel.hansel@...ux.vnet.ibm.com>,
	"Jason J. Herne" <jjherne@...ux.vnet.ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	Andreas Faerber <afaerber@...e.de>
Subject: Re: [Qemu-devel] [PATCH v5 15/17] target-s390x: Extend arch
 specific QMP command query-cpu-definitions

On Mon, 27 Apr 2015 10:11:29 +0200
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> > This patch implements the QMP command 'query-cpu-definitions' in the S390
> > context. The command returns a list of cpu definitions in the current host
> > context. A runnable and migratable cpu model has the related attributes
> > set to true. The order attribute is used to bring the listed cpu definitions
> > in a release order.  
> 
> Can you add some explanation why we need the fallback code (e.g. something
> along the line, when querying no KVM guest is available and to query the
> capabilities we have to open a dummy VM bla bla)

Maybe something like that:

The returned values for attributes like runnable depend on the machine type QEMU is running on.
The function kvm_s390_get_machine_props() is used to determine that. If QEMU was started for
accelerator KVM, a KVMstate is established and machine properties are retrieved by
cpu_model_get(). In case no KVMstate was established, e.g. during QEMU startup in daemonized mode
with the default accelerator TCG, a fallback routine named get_machine_props_fallback() is used
to retrieve the KVM machine properties. It first creates a temporary VM, performs the required
ioctls and finally destroys the VM again.

Michael

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