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

[Igor Mammedov <imammedo@redhat.com>]

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)
> >
> > 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.
> >
> > 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
>