On Mon, 08 Aug 2022 19:57:23 +0200 BB <shen...@gmail.com> wrote: > Am 8. August 2022 14:15:40 MESZ schrieb Igor Mammedov <imamm...@redhat.com>: > >On Wed, 3 Aug 2022 19:26:30 +0200 > >Bernhard Beschow <shen...@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.mayd...@linaro.org> wrote: > >> > > > >> > >> On Thu, 28 Jul 2022 at 12:50, Igor Mammedov <imamm...@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-a2cc52cf0...@redhat.com/ > >> > > >> > https://lore.kernel.org/qemu-devel/20190304210359-mutt-send-email-...@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 <imamm...@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 > >> > >> > >> > > > >> > > >> > > >> > > > >