Re: [PATCH v2 1/2] hw/i386/isapc.c: remove support for -cpu host and -cpu max

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

On Fri, Sep 26, 2025 at 02:49:00PM +0100, Mark Cave-Ayland wrote:
> On 26/09/2025 13:39, Daniel P. Berrangé wrote:
> 
> > On Fri, Sep 26, 2025 at 01:36:25PM +0200, Markus Armbruster wrote:
> > > Mark Cave-Ayland <mark.caveayland@nutanix.com> writes:
> > > 
> > > > Following recent discussions on the mailing list, it has been decided
> > > > that instead of mapping -cpu host and -cpu max to a suitable 32-bit x86 CPU,
> > > > it is preferable to disallow them and use the existing valid_cpu_types
> > > > validation logic so that an error is returned to the user instead.
> > > > 
> > > > Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
> > > > ---
> > > >   hw/i386/isapc.c | 27 ---------------------------
> > > >   1 file changed, 27 deletions(-)
> > > > 
> > > > diff --git a/hw/i386/isapc.c b/hw/i386/isapc.c
> > > > index 44f4a44672..6c35a397df 100644
> > > > --- a/hw/i386/isapc.c
> > > > +++ b/hw/i386/isapc.c
> > > > @@ -41,31 +41,6 @@ static void pc_init_isa(MachineState *machine)
> > > >       DriveInfo *hd[MAX_IDE_BUS * MAX_IDE_DEVS];
> > > >       int i;
> > > > -    /*
> > > > -     * 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");
> > > > -    }
> > > > -
> > > > -    /*
> > > > -     * Similarly if someone unintentionally passes "-cpu host" for the isapc
> > > > -     * machine then display a warning and also switch to the "best" 32-bit
> > > > -     * cpu possible which we consider to be the pentium3. This is because any
> > > > -     * host CPU will already be modern than this, but it also ensures any
> > > > -     * newer CPU flags/features are filtered out for older guests.
> > > > -     */
> > > > -    if (!strcmp(machine->cpu_type, X86_CPU_TYPE_NAME("host"))) {
> > > > -        machine->cpu_type = X86_CPU_TYPE_NAME("pentium3");
> > > > -        warn_report("-cpu host is invalid for isapc machine, using pentium3");
> > > > -    }
> > > > -
> > > >       if (machine->ram_size > 3.5 * GiB) {
> > > >           error_report("Too much memory for this machine: %" PRId64 " MiB, "
> > > >                        "maximum 3584 MiB", machine->ram_size / MiB);
> > > > @@ -162,8 +137,6 @@ static void isapc_machine_options(MachineClass *m)
> > > >           X86_CPU_TYPE_NAME("pentium2"),
> > > >           X86_CPU_TYPE_NAME("pentium3"),
> > > >           X86_CPU_TYPE_NAME("qemu32"),
> > > > -        X86_CPU_TYPE_NAME("max"),
> > > > -        X86_CPU_TYPE_NAME("host"),
> > > >           NULL
> > > >       };
> > > >       PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> > > 
> > > This reverts the "smart" part of recent
> > > 
> > > commit e1e2909f8e74051a34a044940f90d4650b6e784a
> > > Author: Mark Cave-Ayland <mark.caveayland@nutanix.com>
> > > Date:   Thu Aug 28 12:09:44 2025 +0100
> > > 
> > >      hw/i386/pc_piix.c: restrict isapc machine to 32-bit CPUs
> > >      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 either -cpu max or
> > >      -cpu host then the "best" 32-bit CPU is used (in this case the pentium3).
> > 
> > What is written here made sense from the POV of use of isapc with
> > qemu-system-x86_64, but in qemu-system-i686, both 'max' and 'host'
> > where already 32-bit CPUs IIUC. Both this original patch and
> > the new patch block them from being used in qemu-system-i686
> > which feels wrong given the justification above.
> 
> I tried stepping through with -cpu host/-cpu max on qemu-system-i386 and
> it's a bit confusing: I think we end up with some kind of custom AMD vendor
> CPU but with LM disabled. I can't easily see a way to understand what
> features are currently enabled?
> 
> I must admit I'm struggling to see the usefulness of -cpu host/-cpu max for
> isapc given that older OSs can be quite picky when it comes to hardware.

But x86 CPU vendors go to ridiculous levels of complexity to retain
historical back compat over many decades. If anything, I'd be surprised
about the opposite - an OS that didn't work with -cpu max.


With regards,
Daniel
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