Re: [PATCH for-7.1] hw/mips/malta: turn off x86 specific features of PIIX4_PM

[Philippe Mathieu-Daudé]

On 28/7/22 15:16, Igor Mammedov wrote:
> On Thu, 28 Jul 2022 13:29:07 +0100
> Peter Maydell <peter.maydell@linaro.org> wrote:
>
> > On Thu, 28 Jul 2022 at 12:50, Igor Mammedov <imammedo@redhat.com> wrote:
> > > QEMU crashes trying to save VMSTATE when only MIPS target are compiled in
> > >    $ qemu-system-mips -monitor stdio
> > >    (qemu) migrate "exec:gzip -c > STATEFILE.gz"
> > >    Segmentation fault (core dumped)
> > >
> > > It happens due to PIIX4_PM trying to parse hotplug vmstate structures
> > > which are valid only for x86 and not for MIPS (as it requires ACPI
> > > tables support which is not existent for ithe later)

We already discussed this Frankenstein PIIX4 problem 2 and 4 years ago:
https://lore.kernel.org/qemu-devel/4d42697e-ba84-e5af-3a17-a2cc52cf0dbc@redhat.com/
https://lore.kernel.org/qemu-devel/20190304210359-mutt-send-email-mst@kernel.org/

> > > Issue was probably exposed by trying to cleanup/compile out unused
> > > ACPI bits from MIPS target (but forgetting about migration bits).
> > >
> > > Disable compiled out features using compat properties as the least
> > > risky way to deal with issue.

So now MIPS is forced to use meaningless compat[] to satisfy X86.

Am I wrong seeing this as a dirty hack creeping in, yet another
technical debt that will hit (me...) back in a close future?

Are we sure there are no better solution (probably more time consuming
and involving refactors) we could do instead?

Thanks,

Phil.

> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> >
> > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/995
> >
> > > ---
> > > PS:
> > > another approach could be setting defaults to disabled state and
> > > enabling them using compat props on PC machines (which is more
> > > code to deal with => more risky) or continue with PIIX4_PM
> > > refactoring to split x86-shism out (which I'm not really
> > > interested in due to risk of regressions for not much of
> > > benefit)
> > > ---
> > >   hw/mips/malta.c | 9 +++++++++
> > >   1 file changed, 9 insertions(+)
> > >
> > > diff --git a/hw/mips/malta.c b/hw/mips/malta.c
> > > index 7a0ec513b0..0e932988e0 100644
> > > --- a/hw/mips/malta.c
> > > +++ b/hw/mips/malta.c
> > > @@ -1442,6 +1442,14 @@ static const TypeInfo mips_malta_device = {
> > >       .instance_init = mips_malta_instance_init,
> > >   };
> > >
> > > +GlobalProperty malta_compat[] = {
> > > +    { "PIIX4_PM", "memory-hotplug-support", "off" },
> > > +    { "PIIX4_PM", "acpi-pci-hotplug-with-bridge-support", "off" },
> > > +    { "PIIX4_PM", "acpi-root-pci-hotplug", "off" },
> > > +    { "PIIX4_PM", "x-not-migrate-acpi-index", "true" },
> > > +};
> >
> > Is there an easy way to assert in hw/acpi/piix4.c that if
> > CONFIG_ACPI_PCIHP was not set then the board has initialized
> > all these properties to the don't-use-hotplug state ?
> > That would be a guard against similar bugs (though I suppose
> > we probably aren't likely to add new piix4 boards...)
>
> unfortunately new features still creep in 'pc' machine
> ex: "acpi-root-pci-hotplug"), and I don't see an easy
> way to compile that nor enforce that in the future.
>
> Far from easy would be split piix4_pm on base/enhanced
> classes so we wouldn't need x86 specific hacks in 'base'
> variant (assuming 'enhanced' could maintain the current
> VMSTATE to keep cross-version migration working).
>
> > > +const size_t malta_compat_len = G_N_ELEMENTS(malta_compat);
> > > +
> > >   static void mips_malta_machine_init(MachineClass *mc)
> > >   {
> > >       mc->desc = "MIPS Malta Core LV";
> > > @@ -1455,6 +1463,7 @@ static void mips_malta_machine_init(MachineClass *mc)
> > >       mc->default_cpu_type = MIPS_CPU_TYPE_NAME("24Kf");
> > >   #endif
> > >       mc->default_ram_id = "mips_malta.ram";
> > > +    compat_props_add(mc->compat_props, malta_compat, malta_compat_len);
> > >   }
> > >
> > >   DEFINE_MACHINE("malta", mips_malta_machine_init)
> > > --
> > > 2.31.1
> >
> > thanks
> > -- PMM