Re: [Qemu-devel] [RFC PATCH 1/3] target-i386: Add 486sx, old486, and old486sx CPU models

[Eduardo Habkost]

On Mon, Mar 25, 2013 at 04:15:15PM +0100, Igor Mammedov wrote:
> On Mon, 25 Mar 2013 10:39:36 +0100
> Andreas Färber <address@hidden> wrote:
> 
> > Am 28.02.2013 03:12, schrieb H. Peter Anvin:
> > > From: "H. Peter Anvin" <address@hidden>
> > > 
> > > Add models for 486SX, and pre-CPUID versions of the 486 (DX & SX).
> > > Change the model number for the standard 486DX to a model which
> > > actually had CPUID.
> > > 
> > > Note: these models are fairly vestigial, for example most of the FPU
> > > operations still work; only F*ST[CS]W have been modified to appear as
> > > through there is no FPU.
> > > 
> > > This also changes the classic 486 model number to 8 (DX4) which
> > > matches the feature set presented.
> > > 
> > > Signed-off-by: H. Peter Anvin <address@hidden>
[...]
> > > +    },
> > > +    {
> > >          .name = "486",
> > >          .level = 1,
> > >          .vendor = CPUID_VENDOR_INTEL,
> > >          .family = 4,
> > > -        .model = 0,
> > > +        .model = 8,
> > 
> > Such changes have been rejected in the past (e.g., n270 Atom).
> > I personally wouldn't object to 486 changes, but I guess it should
> > rather be handled via Igor's CPU static properties that I have in my
> > review queue: The .model value would be set to 8 but the PC machine
> > would be changed alongside to set model = 0 for pc-1.4 and earlier.
> It doesn't relates to property refactoring nor to slim CPU sub-classes
> conversion either. So it could go in independently.
> 
> But is this change safe from migration POV?

Migration is exactly the reason we can't include this as-is, and where
having static properties would be useful. We need to keep model=0 on the
pc-1.3 and older machine-types, and set model=8 only on pc-1.4 and
newer.

With static properties we can simply set it using the compat_props
table; without static properties, we need a compatibility function or
global variable to enable the model=0 behavior.

> 
> > 
> > >          .stepping = 0,
> > >          .features = I486_FEATURES,
> > >          .xlevel = 0,
> > >      },

-- 
Eduardo