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

[Mark Cave-Ayland <mark.caveayland@nutanix.com>]

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.


ATB,

Mark.