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

[Igor Mammedov]

On Mon, 08 Aug 2022 19:57:23 +0200
BB <shentey@gmail.com> wrote:

> Am 8. August 2022 14:15:40 MESZ schrieb Igor Mammedov <imammedo@redhat.com>:
> >On Wed, 3 Aug 2022 19:26:30 +0200
> >Bernhard Beschow <shentey@gmail.com> wrote:
> >  
> >> On Tue, Aug 2, 2022 at 8:37 AM Philippe Mathieu-Daudé via <
> >> qemu-devel@nongnu.org> wrote:
> >>   
> >> > 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/
> >> >     
> >> 
> >> 
> >> Interesting reads!
> >> 
> >>   
> >> > >>> 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?
> >> >    
> >> 
> >> Working on the consolidation of piix3 and -4 soutbridges [1] I've stumbled
> >> over certain design decisions where board/platform specific assumptions are
> >> baked into the piix device models. I figure that's the core of the issue.
> >> 
> >> In our case the ACPI functionality is implemented by inheritance while
> >> perhaps it should be implemented using composition. With composition, the
> >> ACPI functionality could be injected by the caller: The pc board would
> >> inject it while the Malta board wouldn't. This would solve both the crash
> >> and above design problem.  
> >
> >While refactoring we should keep migration stream compatible with older
> >QEMU versions (we must not regress widely x86 code path). Which might be
> >tricky in this case.  
> 
> Does this particular fix make future compatibility harder or easier or is it 
> that hard already? IIUC it omits the hotplug bits in the vm state for Malta 
> which is what one would expect there, right?
> 
> >Perhaps the best we could do is follow up on Philippe's idea to make
> >PIIX4_PM frankenstein x86-specific (the least chance for regressions)
> >and create/use clean version for anything else.  
> 
> Having two implementations of the same device means that we'll end up having 
> duplicate code with board/platform-specific assumptions baked in. I guess 
> what Phil cares about is a sustainable solution without hacks that doesn't 
> cause bloat and/or regressions for MIPS, especially for features where MIPS 
> doesn't benefit from. I believe that composition could be such a solution.

maybe creating PIIX4_PM-base without carrying any VMstate code and then
inheriting/branching that into piix4_pm-speced and PIIX4_PM, which will
carry it's own VMstate descriptors (with minimal duplication or somewhat
shared) can be made to work.

> My consolidation work could actually make PIIX4 an option for the PC machine. 
> This means that PIIX4_PM wouldn't be Frankenstein any more. This works 
> already on my branch - for both PC and Malta. Furthermore, it looks like it 
> allowed Malta to benefit more from KVM virtualization, but that's off-topic 
> in this discussion.
> 
> >> I'd be willing to implement it but can't make any promises about the time
> >> frame since I'm currently doing this in my free time. Any hints regarding
> >> the implementation would be welcome, though.
> >> 
> >> Best regards,
> >> Bernhard
> >> 
> >> [1] https://github.com/shentok/qemu/commits/piix-consolidate
> >> 
> >>   
> >> > 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
> >> > >>    
> >> > >    
> >> >
> >> >
> >> >    
> >  
>