Re: [Qemu-devel] [PATCH v1 2/3] object.c: object_class_dynamic_cast return NULL if the class has no type

[John Snow]

On 08/26/2015 05:02 PM, Peter Crosthwaite wrote:
> On Wed, Aug 26, 2015 at 1:36 PM, Alistair Francis
> <address@hidden> wrote:
>> On Tue, Aug 25, 2015 at 12:43 AM, Peter Crosthwaite
>> <address@hidden> wrote:
>>> On Mon, Aug 24, 2015 at 4:36 PM, Alistair Francis
>>> <address@hidden> wrote:
>>>> On Mon, Aug 17, 2015 at 4:37 PM, Peter Crosthwaite
>>>> <address@hidden> wrote:
>>>>> On Mon, Aug 17, 2015 at 3:33 PM, Andreas Färber <address@hidden> wrote:
>>>>>> Am 18.08.2015 um 00:24 schrieb Alistair Francis:
>>>>>>> On Sat, Aug 15, 2015 at 2:22 PM, Peter Crosthwaite
>>>>>>> <address@hidden> wrote:
>>>>>>>> On Mon, Jul 27, 2015 at 11:37 AM, Alistair Francis
>>>>>>>> <address@hidden> wrote:
>>>>>>>>> If the ObjectClass has no type return NULL instead of trying to 
>>>>>>>>> compare
>>>>>>>>> the type name.
>>>>>>>>>
>>>>>>>>
>>>>>>>> What was the issue?
>>>>>>>
>>>>>>> There is a seg fault in object_class_dynamic_cast() because there is
>>>>>>> no type in the ObjectClass struct.
>>>>>>
>>>>>> That should never happen, ever since TYPE_OBJECT is no longer NULL.
>>>>>>
>>>>>>> It happens when it is trying to cast the "pci-device", which is called
>>>>>>> from the ahci_irq_lower() function. The function is testing if the
>>>>>>> device is a pci device, so it should return NULL if it isn't valid.
>>>>>
>>>>> Yes so I vaguely remember this now. It is about MSI interrupts which
>>>>> have nothing to do with sysbus implementation. My solution was to rip
>>>>> that PCI specific stuff out of AHCI completely in my branch. Should
>>>>> sysbus and PCI AHCI classes install their own separate logic for this
>>>>> part via a virtualised hook?
>>>>>
>>>>> On the topic though, I notice many PCI devices have this MSI specific
>>>>> logic in them. Is it possible for devs to just treat interrupts as
>>>>> pins and the PCI layers do the MSI vs non-MSI logic switch in core
>>>>> layers?
>>>>>
>>>>> If Andreas' idea don't work this is still a core QOM bug though. I
>>>>> think object_dynamic_cast should not have this segfault when passed a
>>>>> non implementing object.
>>>>>
>>>>> Regards,
>>>>> Peter
>>>>>
>>>>>>
>>>>>> It rather sounds as if some build-time dependency is wrong, which we
>>>>>> used to run into for the Container type before Paolo macrofied this.
>>>>>>
>>>>>> Please try again with a clean build - if it still occurs, we'll need a
>>>>>> reproducible test case to investigate what is going on rather than
>>>>>> papering over a latent bug.
>>>>
>>>> Hey,
>>>>
>>>> Sorry abut the delay, but I didn't get a chance to look at this last
>>>> week. I tried with a clean setup and still see the seg fault.
>>>>
>>>> I will try to look into it more this week, but if anyone is interested
>>>> here are the steps to reproduce:
>>>>
>>>> On the latest mainline QEMU, with my 2nd and 3rd patches applied
>>>> $ ./configure --target-list="aarch64-softmmu,microblazeel-softmmu"
>>>> --disable-pie --disable-sdl --disable-werror # This is what is
>>>> required at work
>>>> $ ./aarch64-softmmu/qemu-system-aarch64 -M xlnx-ep108 -display none
>>>> -kernel ./u-boot.elf -m 8000000 -nographic -serial mon:stdio # Boot
>>>> u-boot on QEMU
>>>>
>>>> The image I'm using is available at: http://1drv.ms/1NxDXLo
>>>>
>>>
>>> So it's not a core bug. That container_of in ahci_lower_irq is
>>> incorrectly assuming that the passed AHCIState * is always for a PCI,
>>> which it is not in the sysbus case. So it's incorrectly getting the
>>> offset of QOM the object and the QOM cast is treating some invalid
>>> offset into the (or past) object as a QOM object base address.
>>>
>>> The simplest solution is a back pointer in AHCIState to the
>>> encapsulating device (would be a DeviceState *). The container_of is
>>> replaced with a nav of this pointer and then the conditional PCI cast
>>> can work.
>>
>> This seems to fix the problem.
> 
> I assume you have the appropriate setters for the new variable
> elsewhere in the code as well?
> 
>> It seems hacky though, I can't find a
>> better way to check the validity of the PCIDevice. Any ideas?
>>
> 
> So there a few problems in the way of a correct solution. The caller
> for ahci_lower_irq does not have access to the QOM object pointer,
> it's been abstracted away by AHCIState (which is not a QOM object). So
> you would need to replumb the call path to ahci_lower_irq to pass the
> QOM object. This would let you drop the container_of completely.
> 
> The next step would be to virtualise ahci_lower_irq, as this is
> implementation dependent (assume specific devices really do need to
> control the use of PCI MSI?), one implementation for sysbus, one for
> PCI. This is blocked by the re-plumbing described above as the
> virtualised called itself will need a ptr to the QOM object.
> 
> But I think the back ptr is an acceptable solution for the meantime,
> this is a clear bug in Sysbus AHCI and should probably even go to
> qemu-stable.
> 

I'm not intricately familiar with how the QOM plumbing works, but I can
definitely see how assuming all AHCIState pointers come from
AHCIPCIState is a problem...

For the uninitiated, how does MSI work with Sysbus? What does a Sysbus
AHCI device look like to a guest, and what happens if it tries to
utilize the functionality?

(Well, segfault, I guess.)

If someone wants to clue in the device model newbie and send a patch my
way, I'll take it.

--js

>> diff --git a/hw/ide/ahci.c b/hw/ide/ahci.c
>> index 02d85fa..77e58a9 100644
>> --- a/hw/ide/ahci.c
>> +++ b/hw/ide/ahci.c
>> @@ -137,8 +137,11 @@ static void ahci_irq_raise(AHCIState *s, AHCIDevice 
>> *dev)
>>  static void ahci_irq_lower(AHCIState *s, AHCIDevice *dev)
>>  {
>>      AHCIPCIState *d = container_of(s, AHCIPCIState, ahci);
>> -    PCIDevice *pci_dev =
>> -        (PCIDevice *)object_dynamic_cast(OBJECT(d), TYPE_PCI_DEVICE);
>> +    PCIDevice *pci_dev = NULL;
>> +
>> +    if (s->parent_obj) {
> 
> I would make the parent obj compulsory for all AHCIState
> implementations and drop the NULL guard.
> 
>> +        pci_dev = PCI_DEVICE(d);
>> +    }
>>
>>      DPRINTF(0, "lower irq\n");
>>
>> diff --git a/hw/ide/ahci.h b/hw/ide/ahci.h
>> index c055d6b..ac7d2de 100644
>> --- a/hw/ide/ahci.h
>> +++ b/hw/ide/ahci.h
>> @@ -287,6 +287,8 @@ struct AHCIDevice {
>>  };
>>
>>  typedef struct AHCIState {
>> +    DeviceState *parent_obj;
> 
> This name is really for QOM inline parents. We decided a while back to
> use "parent" for the QOM parents and "container" for non-parental
> containers. Memory regions use the .container field for a similar
> purpose.
> 
> Regards,
> Peter
> 
>> +
>>      AHCIDevice *dev;
>>      AHCIControlRegs control_regs;
>>      MemoryRegion mem;
>>
>> Thanks,
>>
>> Alistair
>>
>>>
>>> Regards,
>>> Peter
>>>
>>>> Thanks,
>>>>
>>>> Alistair
>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Andreas
>>>>>>
>>>>>> --
>>>>>> SUSE Linux GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
>>>>>> GF: Felix Imendörffer, Jane Smithard, Graham Norton; HRB 21284 (AG 
>>>>>> Nürnberg)
>>>>>
>>>