Re: [PATCH] acpi: Bodge acpi_index migration

[Igor Mammedov <imammedo@redhat.com>]

On Wed,  6 Apr 2022 09:35:31 +0100
"Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:

> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> The 'acpi_index' field is a statically configured field, which for
> some reason is migrated; this never makes much sense because it's
> command line static.

that's true only for the field that's part of PCIDEvice,
however AcpiPciHpState::acpi_index is runtime state and _must_
be migrated if set, otherwise guest might get wrong index
if it's in process of querying it

 
> However, on piix4 it's conditional, and the condition/test function
> ends up having the wrong pointer passed to it (it gets a PIIX4PMState
> not the AcpiPciHpState it was expecting, because VMSTATE_PCI_HOTPLUG
> is a macro and not another struct).  This means the field is randomly
> loaded/saved based on a random pointer.  In 6.x this random pointer
> randomly seems to get 0 for everyone (!); in 7.0rc it's getting junk
> and trying to load a field that the source didn't send.  The migration
> stream gets out of line and hits the section footer.

I'm a bit confused by description,
do you have a reproducer for me to try?

> The bodge is on piix4 never to load the field:
>   a) Most 6.x builds never send it, so most of the time the migration
>     will work.
>   b) We can backport this fix to 6.x to remove the boobytrap.
>   c) It should never have made a difference anyway since the acpi-index
>     is command line configured and should be correct on the destination
>     anyway
>   d) ich9 is still sending/receiving this (unconditionally all the time)
>     but due to (c) should never notice.  We could follow up to make it
>     skip.
> 
> It worries me just when (a) actually happens.
> 
> Fixes: b32bd76 ("pci: introduce acpi-index property for PCI device")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/932
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  hw/acpi/acpi-pci-hotplug-stub.c |  4 ----
>  hw/acpi/pcihp.c                 |  6 ------
>  hw/acpi/piix4.c                 | 11 ++++++++++-
>  include/hw/acpi/pcihp.h         |  2 --
>  4 files changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/acpi/acpi-pci-hotplug-stub.c b/hw/acpi/acpi-pci-hotplug-stub.c
> index 734e4c5986..a43f6dafc9 100644
> --- a/hw/acpi/acpi-pci-hotplug-stub.c
> +++ b/hw/acpi/acpi-pci-hotplug-stub.c
> @@ -41,7 +41,3 @@ void acpi_pcihp_reset(AcpiPciHpState *s, bool acpihp_root_off)
>      return;
>  }
>  
> -bool vmstate_acpi_pcihp_use_acpi_index(void *opaque, int version_id)
> -{
> -    return false;
> -}
> diff --git a/hw/acpi/pcihp.c b/hw/acpi/pcihp.c
> index 6351bd3424..bf65bbea49 100644
> --- a/hw/acpi/pcihp.c
> +++ b/hw/acpi/pcihp.c
> @@ -554,12 +554,6 @@ void acpi_pcihp_init(Object *owner, AcpiPciHpState *s, PCIBus *root_bus,
>                                     OBJ_PROP_FLAG_READ);
>  }
>  
> -bool vmstate_acpi_pcihp_use_acpi_index(void *opaque, int version_id)
> -{
> -     AcpiPciHpState *s = opaque;
> -     return s->acpi_index;
> -}
> -
>  const VMStateDescription vmstate_acpi_pcihp_pci_status = {
>      .name = "acpi_pcihp_pci_status",
>      .version_id = 1,
> diff --git a/hw/acpi/piix4.c b/hw/acpi/piix4.c
> index cc37fa3416..48aeedd5f0 100644
> --- a/hw/acpi/piix4.c
> +++ b/hw/acpi/piix4.c
> @@ -267,6 +267,15 @@ static bool piix4_vmstate_need_smbus(void *opaque, int version_id)
>      return pm_smbus_vmstate_needed();
>  }
>  
> +/*
> + * This is a fudge to turn off the acpi_index field, whose
> + * test was always broken on piix4.
> + */
> +static bool vmstate_test_never(void *opaque, int version_id)
> +{
> +    return false;
> +}
> +
>  /* qemu-kvm 1.2 uses version 3 but advertised as 2
>   * To support incoming qemu-kvm 1.2 migration, change version_id
>   * and minimum_version_id to 2 below (which breaks migration from
> @@ -297,7 +306,7 @@ static const VMStateDescription vmstate_acpi = {
>              struct AcpiPciHpPciStatus),
>          VMSTATE_PCI_HOTPLUG(acpi_pci_hotplug, PIIX4PMState,
>                              vmstate_test_use_acpi_hotplug_bridge,
> -                            vmstate_acpi_pcihp_use_acpi_index),
> +                            vmstate_test_never),
>          VMSTATE_END_OF_LIST()
>      },
>      .subsections = (const VMStateDescription*[]) {
> diff --git a/include/hw/acpi/pcihp.h b/include/hw/acpi/pcihp.h
> index af1a169fc3..7e268c2c9c 100644
> --- a/include/hw/acpi/pcihp.h
> +++ b/include/hw/acpi/pcihp.h
> @@ -73,8 +73,6 @@ void acpi_pcihp_reset(AcpiPciHpState *s, bool acpihp_root_off);
>  
>  extern const VMStateDescription vmstate_acpi_pcihp_pci_status;
>  
> -bool vmstate_acpi_pcihp_use_acpi_index(void *opaque, int version_id);
> -
>  #define VMSTATE_PCI_HOTPLUG(pcihp, state, test_pcihp, test_acpi_index) \
>          VMSTATE_UINT32_TEST(pcihp.hotplug_select, state, \
>                              test_pcihp), \