Re: [Qemu-devel] [RFC qom-cpu v2 2/2] target-i386: Turn Haswell into subclass of SandyBridge

[Eduardo Habkost]

On Wed, Dec 12, 2012 at 03:47:49PM +0100, Andreas Färber wrote:
> Am 12.12.2012 13:45, schrieb Eduardo Habkost:
> > On Mon, Dec 10, 2012 at 11:59:32PM +0100, Andreas Färber wrote:
> >>   ehabkost: "When adding the Haswell CPU model, I intended to make it
> >>   a superset of the features present on the SandyBridge model"
> >>
> >> Inherit from SandyBridge to keep only the delta for Haswell.
> > 
> > Most CPUs have a superset of the features of their predecessors. Are you
> > simply using SandyBridge->Haswell as an example, or you think their
> > relationship is special somehow?
> > 
> > I believe we don't want to make externally-visible class inheritance,
> > but probably just reuse constans or init functions internally. A Haswell
> > CPU is not a type of SandyBridge CPU, it just happens to contain a
> > superset of the features present in SandyBridge.
> > 
> > I mean: Haswell also has a superset of features of 486, but we don't
> > want to make the hierarchy look like the following, do we?
> 
> I don't see why we would want to use a #define-based inheritence as
> suggested for the PPRO when we have QOM. QOM inheritence reduces lines
> of code significantly compared to just taking the values from elsewhere.

The reuse doesn't need to be #define-based (although maybe a
#define-based approach would work too), it could be function-call-based.

> 
> For the Haswell you said what I quoted, for the other models I said I
> need your or someone's help to verify whether a hierarchy such as below
> is semantically right or just a coincidence. I was at least considering
> an abstract intel-/amd-*-cpu to avoid repeating the three value
> assignments over and over.

Creating X86IntelCPU and X86AMDCPU classes make sense to me, because
Haswell is a kind of Intel CPu. Making Haswell a subclass of 486 (like
below) wouldn't.

> 
> At this time I believe the parents of a type are not (yet) exposed via
> QMP, just the "type" string property.

Even if they are not exposed externally, it's a confusing usage of
inheritance for me. I mean: a Haswell CPU is not a type of 486 CPU, it's
simply a different kind of CPU that happens to have a superset of the
486 features.

> 
> Andreas
> 
> > - X86CPU
> >   -> X86IntelCPU
> >      -> 486
> >         -> pentium
> >            -> pentium2
> >               -> pentium3
> >                  -> Conroe
> >                     -> Penryn
> >                        -> Nehalem
> >                           -> Westmere
> >                              -> SandyBridge
> >                                 -> Haswell
> 
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-- 
Eduardo