Re: [Qemu-devel] [RFC PATCH v1 05/10] cpu: Abstract CPU core type

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Mar 07, 2016 at 11:40:11AM +0100, Igor Mammedov wrote:
> On Mon, 7 Mar 2016 14:01:55 +0530
> Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> 
> > On Mon, Mar 07, 2016 at 02:36:55PM +1100, David Gibson wrote:
> > > On Fri, Mar 04, 2016 at 07:07:20PM +0100, Igor Mammedov wrote:  
> > > > On Fri, 4 Mar 2016 16:32:53 +0530
> > > > Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> > > >   
> > > > > On Fri, Mar 04, 2016 at 11:38:45AM +0100, Igor Mammedov wrote:  
> > > > > > On Fri,  4 Mar 2016 12:24:16 +0530
> > > > > > Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> > > > > >     
> > > > > > > Add an abstract CPU core type that could be used by machines that want
> > > > > > > to define and hotplug CPUs in core granularity.
> > > > > > > 
> > > > > > > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > > > > > > ---
> > > > > > >  hw/cpu/Makefile.objs  |  1 +
> > > > > > >  hw/cpu/core.c         | 44 ++++++++++++++++++++++++++++++++++++++++++++
> > > > > > >  include/hw/cpu/core.h | 30 ++++++++++++++++++++++++++++++
> > > > > > >  3 files changed, 75 insertions(+)
> > > > > > >  create mode 100644 hw/cpu/core.c
> > > > > > >  create mode 100644 include/hw/cpu/core.h
> > > > > > > 
> > > > > > > diff --git a/hw/cpu/Makefile.objs b/hw/cpu/Makefile.objs
> > > > > > > index 0954a18..942a4bb 100644
> > > > > > > --- a/hw/cpu/Makefile.objs
> > > > > > > +++ b/hw/cpu/Makefile.objs
> > > > > > > @@ -2,4 +2,5 @@ obj-$(CONFIG_ARM11MPCORE) += arm11mpcore.o
> > > > > > >  obj-$(CONFIG_REALVIEW) += realview_mpcore.o
> > > > > > >  obj-$(CONFIG_A9MPCORE) += a9mpcore.o
> > > > > > >  obj-$(CONFIG_A15MPCORE) += a15mpcore.o
> > > > > > > +obj-y += core.o
> > > > > > >  
> > > > > > > diff --git a/hw/cpu/core.c b/hw/cpu/core.c
> > > > > > > new file mode 100644
> > > > > > > index 0000000..d8caf37
> > > > > > > --- /dev/null
> > > > > > > +++ b/hw/cpu/core.c
> > > > > > > @@ -0,0 +1,44 @@
> > > > > > > +/*
> > > > > > > + * CPU core abstract device
> > > > > > > + *
> > > > > > > + * Copyright (C) 2016 Bharata B Rao <bharata@linux.vnet.ibm.com>
> > > > > > > + *
> > > > > > > + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> > > > > > > + * See the COPYING file in the top-level directory.
> > > > > > > + */
> > > > > > > +#include "hw/cpu/core.h"
> > > > > > > +
> > > > > > > +static char *core_prop_get_slot(Object *obj, Error **errp)
> > > > > > > +{
> > > > > > > +    CPUCore *core = CPU_CORE(obj);
> > > > > > > +
> > > > > > > +    return g_strdup(core->slot);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static void core_prop_set_slot(Object *obj, const char *val, Error **errp)
> > > > > > > +{
> > > > > > > +    CPUCore *core = CPU_CORE(obj);
> > > > > > > +
> > > > > > > +    core->slot = g_strdup(val);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static void cpu_core_instance_init(Object *obj)
> > > > > > > +{
> > > > > > > +    object_property_add_str(obj, "slot", core_prop_get_slot, core_prop_set_slot,
> > > > > > > +                            NULL);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static const TypeInfo cpu_core_type_info = {
> > > > > > > +    .name = TYPE_CPU_CORE,
> > > > > > > +    .parent = TYPE_DEVICE,
> > > > > > > +    .abstract = true,
> > > > > > > +    .instance_size = sizeof(CPUCore),
> > > > > > > +    .instance_init = cpu_core_instance_init,
> > > > > > > +};
> > > > > > > +
> > > > > > > +static void cpu_core_register_types(void)
> > > > > > > +{
> > > > > > > +    type_register_static(&cpu_core_type_info);
> > > > > > > +}
> > > > > > > +
> > > > > > > +type_init(cpu_core_register_types)
> > > > > > > diff --git a/include/hw/cpu/core.h b/include/hw/cpu/core.h
> > > > > > > new file mode 100644
> > > > > > > index 0000000..2daa724
> > > > > > > --- /dev/null
> > > > > > > +++ b/include/hw/cpu/core.h
> > > > > > > @@ -0,0 +1,30 @@
> > > > > > > +/*
> > > > > > > + * CPU core abstract device
> > > > > > > + *
> > > > > > > + * Copyright (C) 2016 Bharata B Rao <bharata@linux.vnet.ibm.com>
> > > > > > > + *
> > > > > > > + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> > > > > > > + * See the COPYING file in the top-level directory.
> > > > > > > + */
> > > > > > > +#ifndef HW_CPU_CORE_H
> > > > > > > +#define HW_CPU_CORE_H
> > > > > > > +
> > > > > > > +#include "qemu/osdep.h"
> > > > > > > +#include "hw/qdev.h"
> > > > > > > +
> > > > > > > +#define TYPE_CPU_CORE "cpu-core"
> > > > > > > +
> > > > > > > +#define CPU_CORE(obj) \
> > > > > > > +    OBJECT_CHECK(CPUCore, (obj), TYPE_CPU_CORE)
> > > > > > > +
> > > > > > > +typedef struct CPUCore {
> > > > > > > +    /*< private >*/
> > > > > > > +    DeviceState parent_obj;
> > > > > > > +
> > > > > > > +    /*< public >*/
> > > > > > > +    char *slot;
> > > > > > > +} CPUCore;
> > > > > > > +
> > > > > > > +#define CPU_CORE_SLOT_PROP "slot"    
> > > > > > as it's generic property I'd rename to 'core' so it would fit all users    
> > > > > 
> > > > > Ok. Also note that this is a string property which is associated with the
> > > > > link name (string) that we created from machine object to this core. I think
> > > > > it would be ideal if this becomes an interger  property in which case it
> > > > > becomes easier to feed the core location into your CPUSlotProperties.core.  
> > > > agreed, it should be core number.  
> > > 
> > > The slot stuff is continuing to confuse me a bit.  I see that we need
> > > some kind of "address" value, but how best to do it is not clear to
> > > me.
> > > 
> > > Changing this to an integer sounds like it's probably a good idea.
> > > I'm a bit wary of just calling it "core" though.  Do all platforms
> > > even necessarily have a core id?
> > > 
> > > I'm wondering if the addressing is something that needs to move the
> > > the platform specific subtypes, while some other stuff can move to the
> > > generic base type.
> > >   
> > > > > > on top of that I'd add numeric 'threads' property to base class so
> > > > > > all derived cores would inherit it.
> > > > > > 
> > > > > > Then as easy integration with -smp threads=x, a machine could push
> > > > > > a global variable 'cpu-core.threads=[smp_threads]' which would
> > > > > > make every created cpu-core object to have threads set
> > > > > > at instance_init() time (device_init).
> > > > > > 
> > > > > > That way user won't have to specify 'threads=y' for every
> > > > > >   device_add spapr-core,core=x
> > > > > > as it will be taken from global property 'cpu-core.threads'
> > > > > > but if user wishes he/she still could override global by explicitly
> > > > > > providing thread property at device_add time:
> > > > > >   device_add spapr-core,core=x,threads=y
> > > > > > 
> > > > > > wrt this series it would mean, instead of creating threads in property
> > > > > > setter, delaying threads creation to core.realize() time,
> > > > > > but since realize is allowed to fail it should be fine do so.    
> > > > > 
> > > > > Ok that would suit us as there are two properties on which thread creation
> > > > > is dependent upon: nr_threads and cpu_model. If thread objects can be
> > > > > created at core realize time, then we don't have to resort to the ugliness
> > > > > of creating the threads from either of the property setters. I always
> > > > > assumed that we shouldn't be creating objects from realize, but if that
> > > > > is fine, it is good.  
> > > > since realize is allowed to fail, it should be safe from hotplug pov
> > > > to create internal objects there, as far as proper cleanups are done
> > > > for failure path.  
> > > 
> > > Right, moving the "nr_threads" property to the base type seems like a
> > > good idea to me.  
> > 
> > And we will also move the cpu_model property (now being tracked by
> > an ObjectClass pointer) to the base type ?
> I'm not sure that moving cpu_model to the base class is the right thing,
> I'd keep it local to platform for now.

I tend to agree, although I'm not sure that I could really explain why
:/

> Could you have several spapr core types? One per CPU model?
> That way you won't need to track cpu_model when using device_add.

We could in theory, but it would be pretty inconvenient.  Because this
is a paravirt platform, there really can't be any core-level
difference between them, and it would mean creating a fair batch of
core types for the various minor POWER7 and POWER8 variants - and
needing to update this whenever IBM makes a new version.  I suspect it
would also introduce more wrinkles in order to have a correct
"spapr-core-host" type matching the "HOST" cpu thread type.  Since KVM
(HV) only supports the HOST thread type, that's a fairly big issue.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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