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

[Michael Mueller <mimu@linux.vnet.ibm.com>]

On Mon, 27 Apr 2015 10:11:29 +0200
Christian Borntraeger <borntraeger@de.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