On Mon, 22 Sep 2025 14:56:57 +0100 Mark Cave-Ayland <[email protected]> wrote:
> On 22/09/2025 13:35, Igor Mammedov wrote: > > > On Mon, 22 Sep 2025 14:05:13 +0200 > > Philippe Mathieu-Daudé <[email protected]> wrote: > > > >> On 27/8/25 13:46, Daniel P. Berrangé wrote: > >>> 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 <[email protected]> > >>>>>> Reviewed-by: Philippe Mathieu-Daudé <[email protected]> > >>>>>> --- > >>>>>> 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. > >> > >> OK, on 2nd thought, even while warning the user, changing the type > >> under the hood isn't great. > > > > I second that, > > Please don't do magical mutations of CPUs, just error out. > > > > we used to 'fix|tweak' CPUs using machine compat hack, > > however with introduction of versioned cpu models we shouldn't do that > > anymore. > > (aka: existing CPU devices should stay immutable if possible, and any > > visible > > changes should go into new version) > > The original suggestion for allowing "max"/"host" was so that it > wouldn't cause any regressions with command lines erroneously including > -cpu max or -cpu host (which I believe may be possible with libvirt). looking back and at Daniels reply, max/host are indeed are 'special' aka mutable as opposed to named cpu models. if we go by the books, 'host' and by extension 'max' should work with KVM accelerator. But that (aka reducing it to isapc levels) should be done at 'host' cpu model code and that part of code is not really aware (nor should be) of machine types. I'm not sure, whether it's worth the effort and complexity. I'd be fine with valid_cpu_types[] approach here, i.e. user will get clear error that her is doing wrong thing trying 'host/max', and printing suggestion how to remedy error should guide user to the right config. > > >> What about simply removing "max" of valid_cpu_types[], since it is > >> clearly confusing "max" == "pentium3"... > > > > it seems that specifying supported cpu models in valid_cpu_types[] is the > > way to go. > > That was what I did in v1 and v2 version of the series, but I can submit > a patch to change this once there is agreement on the desired behaviour. > > > ATB, > > Mark. >
