Re: [PATCH v6 01/19] hw/i386/pc_piix.c: restrict isapc machine to 32-bit CPUs

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Aug 27, 2025 at 12:10:00PM +0100, Mark Cave-Ayland wrote:
> On 26/08/2025 08:25, Xiaoyao Li wrote:
> 
> > On 8/22/2025 8:11 PM, Mark Cave-Ayland wrote:
> > > The isapc machine represents a legacy ISA PC with a 486 CPU. Whilst it is
> > > possible to specify any CPU via -cpu on the command line, it makes no
> > > sense to allow modern 64-bit CPUs to be used.
> > > 
> > > Restrict the isapc machine to the available 32-bit CPUs, taking care to
> > > handle the case where if a user inadvertently uses -cpu max then the
> > > "best"
> > > 32-bit CPU is used (in this case the pentium3).
> > > 
> > > Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
> > > Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> > > ---
> > >   hw/i386/pc_piix.c | 26 ++++++++++++++++++++++++++
> > >   1 file changed, 26 insertions(+)
> > > 
> > > diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> > > index c03324281b..5720b6b556 100644
> > > --- a/hw/i386/pc_piix.c
> > > +++ b/hw/i386/pc_piix.c
> > > @@ -436,6 +436,19 @@ static void pc_set_south_bridge(Object *obj,
> > > int value, Error **errp)
> > >   #ifdef CONFIG_ISAPC
> > >   static void pc_init_isa(MachineState *machine)
> > >   {
> > > +    /*
> > > +     * There is a small chance that someone unintentionally passes
> > > "- cpu max"
> > > +     * for the isapc machine, which will provide a much more modern
> > > 32-bit
> > > +     * CPU than would be expected for an ISA-era PC. If the "max"
> > > cpu type has
> > > +     * been specified, choose the "best" 32-bit cpu possible which
> > > we consider
> > > +     * be the pentium3 (deliberately choosing an Intel CPU given
> > > that the
> > > +     * default 486 CPU for the isapc machine is also an Intel CPU).
> > > +     */
> > > +    if (!strcmp(machine->cpu_type, X86_CPU_TYPE_NAME("max"))) {
> > > +        machine->cpu_type = X86_CPU_TYPE_NAME("pentium3");
> > > +        warn_report("-cpu max is invalid for isapc machine, using
> > > pentium3");
> > > +    }
> > 
> > Do we need to handle the case of "-cpu host"?
> 
> I don't believe so. I wasn't originally planning to support "-cpu max" for
> isapc, however Daniel mentioned that it could possibly be generated from
> libvirt so it makes sense to add the above check to warn in this case and
> then continue.

Libvirt will support sending any valid -cpu flag, including both
'max' (any config) and 'host' (if KVM).

If 'isapc' still expects to support KVM, then it would be odd to
reject 'host', but KVM presumably has no built-in way to limit to
32-bit without QEMU manually masking many features ?

I'm a little worried about implications of libvirt sending '-cpu max'
and QEMU secretly turning that into '-cpu pentium3', as opposed to
having '-cpu max' expand to equiv to 'pentium3', which might cauase
confusion when libvirt queries the expanded CPU ? Copying Jiri for
an opinion from libvirt side, as I might be worrying about nothing.


With regards,
Daniel
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