Re: [PATCH v2 4/5] isa/piix4: Fix PCI IRQ levels to be preserved in VMState

[Bernhard Beschow]

Am 12. Februar 2022 19:30:43 MEZ schrieb Peter Maydell 
<peter.maydell@linaro.org>:
>On Sat, 12 Feb 2022 at 17:02, BALATON Zoltan <balaton@eik.bme.hu> wrote:
>>
>> On Sat, 12 Feb 2022, Peter Maydell wrote:
>> > On Sat, 12 Feb 2022 at 13:42, BALATON Zoltan <balaton@eik.bme.hu> wrote:
>> >> By the way the corresponding member in struct PIIXState in
>> >> include/hw/southbridge/piix.h has a comment saying:
>> >>
>> >>      /* This member isn't used. Just for save/load compatibility */
>> >>      int32_t pci_irq_levels_vmstate[PIIX_NUM_PIRQS];
>> >>
>> >> and only seems to be filled in piix3_pre_save() but never used. So what's
>> >> the point of this then? Maybe piix3 also uses a bitmap to store these
>> >> levels instead? There's a uint64_t pic_levels member above the unused
>> >> array but I haven't checked the implementation.
>> >
>> > I think what has happened here is that originally piix3 used
>> > the same implementation piix4 currently does -- where it stores
>> > locally the value of the (incoming) PCI IRQ levels, and then when it wants
>> > to know the value of the (outgoing) PIC IRQ levels it loops round
>> > to effectively OR together all the PCI IRQ levels for those PCI
>> > IRQs which are configured to that particular PIC interrupt.
>> >
>> > Then in commit e735b55a8c11 (in 2011) piix3 was changed to call
>> > pci_bus_get_irq_level() to get the value of a PCI IRQ rather than
>> > looking at its local cache of that information in the pci_irq_levels[]
>> > array. This is the source of the "save/load compatibility" thing --
>> > before doing a vmsave piix3_pre_save() fills in that field so that
>> > it appears in the outbound data stream and thus a migration from
>> > a new QEMU to an old pre-e735b55a8c11 QEMU will still work. (This
>> > was important at the time, but could probably be cleaned up now.)
>> >
>> > The next commit after that one is ab431c283e7055bcd, which
>> > is an optimization that fixes the equivalent of the "XXX: optimize"
>> > marker in the gt64120_pci_set_irq()/piix4 code -- this does
>> > something slightly more complicated involving the pic_levels
>> > field, in order to avoid having to do the "loop over all the
>> > PCI IRQ levels" whenever it needs to set/reset a PIC interrupt.
>> >
>> > We could pick up one or both (or none!) of these two changes
>> > for the piix4 code. (If we're breaking migration compat anyway
>> > we wouldn't need to include the save-load compat part of
>> > the first change.)
>>
>> I'm not sure I fully get this. Currently (before this series) PIIX4State
>> does not seem to have any fields to store irq state (in hw/isa/piix4.c):
>>
>> struct PIIX4State {
>>      PCIDevice dev;
>>      qemu_irq cpu_intr;
>>      qemu_irq *isa;
>>
>>      RTCState rtc;
>>      /* Reset Control Register */
>>      MemoryRegion rcr_mem;
>>      uint8_t rcr;
>> };
>>
>> Patch 1 in this series introduces that by moving it from MaltaState. Then
>> we could have a patch 2 to clean it up and change to the way piix3 does it
>> and skip introducing the saving of this array into the migration state. It
>> may still break migration but not sure if MaltaState was saved already so
>> this may be already missing from migration anyway and if we do the same as
>> piix3 then maybe we don't need to change the piix4 state either (as this
>> is saved as part of the PCIHost state?) but I don't know much about this
>> so maybe I'm wrong.
>
>Yeah, that would work -- we weren't saving the old gt64xxx_pci.c
>pci_irq_levels[] global, so we don't break anything that wasn't
>already broken by not putting the newly-introduced PIIX4State
>array into migration state. Then when we do the equivalent of
>e735b55a8c11 the array can just be deleted again. (We can't
>conveniently switch to using pci_bus_get_irq_level() before doing
>patch 1 of this series, because we need the pointer to the
>piix4 device object and gt64120_pci_set_irq() is only passed
>a pointer directly to a qemu_irq array.)
>
>> In any case PIIX3 and PIIX4 state seem to be different so there's no
>> reason to worry aobut compatibility between them.
>
>Yep, that's right. The only reasons to copy changes from piix3
>are (a) because they're worth having in themselves and (b)
>because having the two devices be the same is maybe less
>confusing. (b)'s a pretty weak reason, and (a) depends on
>the individual change. In this case I think making the equivalent
>of e735b55a8c11 is worthwhile because it saves us having an
>extra array field and migrating it, and the change is pretty
>small. For ab431c283e7055bcd you could argue either way -- it's
>clearly a better way to structure the irq handling, but it's only
>an optimisation, not a bug fix.

e735b55a8c11 seems like a very elegant way for fixing migration of the IRQ 
levels. I'll have a look.

Regards,
Bernhard
>
>-- PMM