Re: [Qemu-devel] in a device or CPU instance init/realize, can I rely on something having the BQL or equivalent?

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Dec 08, 2017 at 06:32:03PM +0100, Igor Mammedov wrote:
> On Fri, 8 Dec 2017 14:14:22 -0200
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > On Fri, Dec 08, 2017 at 03:50:50PM +0100, Igor Mammedov wrote:
> > > On Fri, 8 Dec 2017 13:19:27 +0000
> > > Peter Maydell <peter.maydell@linaro.org> wrote:
> > >   
> > > > On 8 December 2017 at 13:16, Igor Mammedov <imammedo@redhat.com> wrote:  
> > > > > TBH:
> > > > >  I do not recall why we have x86 max/host cpu types do feature
> > > > >  loading at realize time instead of at class init like the rest
> > > > >  of static cpu types.    
> > > > 
> > > > class init is too early, IIRC -- it's before KVM has been set up at all.  
> > > 
> > > that shouldn't be an issue as kvm_ppc_register_host_cpu_type() demonstrates
> > > (i.e. an additional class init at kvm/tcg init time),  
> > 
> > It is possible, but IMO it's not a good idea.  We should be able
> > to enumerate all CPU types before the accelerator has been
> > initialized, so query-cpu-definitions and "-cpu help" will always
> > work.
> > 
> > 
> > > 
> > > so it might be some compat issue or just legacy approach why it
> > > havn't been rewritten to class_init for x86 the way PPC does.
> > > But Eduardo probably knows better if there is anything left that
> > > prevents using class init there.  
> > 
> > It's the opposite: x86 "host" CPU model used to work the same way
> > as PPC, but we changed it so all classes are registered at
> > type_init()-time.
> Is it for libvirt convenience, so that it would be able to cache
> all supported cpus regardless of whether they would actually work
> or not?

I'd say it's for our own convenience, so that things won't break
if we make QMP available before machine/accel initialization in
the future, or if we reorder the code that handles "-cpu ?" in
main().

Moving code around is simpler if we know that all QOM types are
registered at the same point, so we can treat them as static data
after module_call_init(MODULE_INIT_QOM) runs.

-- 
Eduardo