Re: [Qemu-devel] KVM call minutes for Feb 8

[Anthony Liguori]

On 02/13/2011 09:56 AM, Avi Kivity wrote:
> On 02/13/2011 05:38 PM, Anthony Liguori wrote:
> > > > 2) get rid of the entire concept of machines.  Creating a i440fx is 
> > > > essentially equivalent to creating a bare machine.
> > >
> > > No, it's not.  The 440fx does not include an IOAPIC, for example.  
> > > There may be other optional components, or differences in wiring, 
> > > that make two machines with i440fx not identical.
> >
> >
> > The IOAPIC is basically the only other component and I view it as 
> > part of the CPU interface to the chipset.
>
> But it isn't.  The IOAPIC is not per-core or per-socket.

Yeah, I wasn't implying that it was.

>   It's strictly board level.  It's a board interface to the cpu.
>
> > But still, if we're creating a machine from scratch:
> >
> > qemu -device i440fx,id=nb -device piix3,id=sb,chipset=nb -device 
> > ioapic,id=ioapic,chipset=sb -device cpu,ioapic=ioapic,northbridge=nb
> >
> > Is not all that unreasonable and presents a fully functioning PC.
>
> Sure.  And -M blah is a shortcut.

Exactly.  Or better yet, blah is a config file that contains

[device "nb"]
driver=i440fx

[device "sb"]
driver=piix3
chipset=nb

[device "ioapic"]
driver=ioapic
chipset=sb

[device "cpu"]
driver=cpu
ioapic=ioapic
northbridge=nb

> > > > 4) model the CPUs as devices that take a pointer to a host 
> > > > controller, for x86, the normal case would be giving it a pointer 
> > > > to i440fx.
> > >
> > > Surely the connection is via a bus?  An x86 cpu talks to the bus, 
> > > and there happens to be an 440fx north bridge at the end of it.  It 
> > > could also be a Q35 or something else.
> >
> > I see being on a bus as really just taking a pointer to an 
> > interface.  So yes, the i440fx would implement a PentiumCpuInterface 
> > or something like that and the CPU would take a pointer to a 
> > PentiumCpuInterface[1].
> >
> > This is part of why having proper polymorphism is important.  We need 
> > it in order to be able to express concepts like interfaces.
>
> The CPUs and northbridge are peers on the system bus.  However, this 
> isn't something good to model, since it keeps changing without any 
> guest software impact.

If we can move away from Bus abstraction and to a simpler interface 
mechanism, then we can express peer relationships by just having 
bidirection references.  IOW:

-device cpus,northbridge=nb,id=cpus,count=16 -device i440fx,cpus=cpus

I don't think modelling each CPU makes sense.  We should probably just 
model all cpus in a single device for the sake of simplicity.

Regards,

Anthony Liguori