Re: [PATCH] acpi: Bodge acpi_index migration

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 6 Apr 2022 10:38:51 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Igor Mammedov (imammedo@redhat.com) wrote:
> > 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  
> 
> So this patch only changes the piix4.c version; I'm confused, is there
> a AcpiPciHpState::acpi_index that's runtime setable in there?

> > >          VMSTATE_PCI_HOTPLUG(acpi_pci_hotplug, PIIX4PMState,
                                    ^^^ AcpiPciHpState
> > >                              vmstate_test_use_acpi_hotplug_bridge,
> > > -                            vmstate_acpi_pcihp_use_acpi_index),

hw/acpi/pcihp.c:pci_write():
   s->acpi_index = object_property_get_uint(o, "acpi-index", NULL);

s->acpi_index is runtime value that is supposed to be migrated if it's set
to something other then 0

I may have botched VMSTATE_PCI_HOTPLUG, intent was to migrate
AcpiPciHpState::acpi_index if necessary. But I'm not sure how
if I used correct approach for to migrate an optional value
i.e.  maybe instead of VMSTATE_UINT32_TEST(pcihp.acpi_index, state, test_acpi_index)
I should've used subsection, because destination has no clue if
acpi_index would be transmitted over wire or not?
    
> > > 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?  
> 
> Yeh, see the linked gitlab case command line:
>   https://gitlab.com/qemu-project/qemu/-/issues/932
> 
> ./x86_64-softmmu/qemu-system-x86_64 -M pc-q35-6.2 -m 512 -device virtio-scsi-pci,id=scsihw0,bus=pcie.0,addr=0x5,acpi-index=3 -drive if=none,my.qcow2,format=qcow2,id=drive-scsi0,node-name=scsi0 -device 'scsi-hd,bus=scsihw0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0,id=scsi0'  -nographic
> 
> just migrating from a 6.2 to a head triggers this.

Over here any migration from qemu-6.2 to HEAD at 3d31fe4d662f13c7
fails even without acpi-index, as simple as this:

qemu-system-x86_64-6.2 -M pc-i440fx-6.2  -m 512 -vnc :0 -monitor stdio
(qemu) stop
(qemu) migrate "exec:gzip -c > STATEFILE.gz"

qemu-system-x86_64-7.0 -M pc-i440fx-6.2  -m 512 -vnc :0 -monitor stdio -incoming "exec: gzip -c -d STATEFILE.gz"

(qemu) qemu-system-x86_64-7.0: Missing section footer for 0000:00:01.3/piix4_pm
qemu-system-x86_64-7.0: load of migration failed: Invalid argument


Like you pointed out in gitlab issue, vmstate_acpi_pcihp_use_acpi_index
is broken. Following applied to HEAD should fix immediate issue on destination
reading random value:

diff --git a/hw/acpi/piix4.c b/hw/acpi/piix4.c
index f0b5fac44a..c97db491c8 100644
--- a/hw/acpi/piix4.c
+++ b/hw/acpi/piix4.c
@@ -269,6 +269,11 @@ static bool piix4_vmstate_need_smbus(void *opaque, int version_id)
     return pm_smbus_vmstate_needed();
 }
 
+static bool vmstate_piix4_need_acpi_index(void *opaque, int version_id)
+{
+    PIIX4PMState *s = PIIX4_PM(opaque);
+    return vmstate_acpi_pcihp_use_acpi_index(&(s->acpi_pci_hotplug), version_id);
+}
 /* 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
@@ -299,7 +304,7 @@ 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_piix4_need_acpi_index),
         VMSTATE_END_OF_LIST()
     },
     .subsections = (const VMStateDescription*[]) {


> Dave
> 
> > > 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), \  
> >