Re: [Qemu-ppc] [Qemu-devel] [PATCH 56/72] PPC: e500: Use new MPIC dt format

[Scott Wood]

On 08/09/2012 03:52 PM, Alexander Graf wrote:
> 
> On 09.08.2012, at 22:50, Scott Wood wrote:
> 
>> On 08/09/2012 03:48 PM, Alexander Graf wrote:
>>>
>>> On 09.08.2012, at 00:40, Scott Wood wrote:
>>>
>>>> On 08/08/2012 04:16 PM, Alexander Graf wrote:
>>>>>
>>>>> On 24.06.2012, at 01:07, Alexander Graf wrote:
>>>>>
>>>>>> Due to popular demand, we're updating the way we generate the MPIC
>>>>>> node and interrupt lines based on what the current state of art is.
>>>>>>
>>>>>> Requested-by: Scott Wood <address@hidden>
>>>>>> Signed-off-by: Alexander Graf <address@hidden>
>>>>>
>>>>> Hey Scott,
>>>>>
>>>>> This patch breaks SMP for me. The reason for the breakage is that
>>>>> Linux does some things differently when it finds an fsl,mpic instead
>>>>> of a generic openpic. I have assembled logs between a working version
>>>>> (compatible openpic) and a broken version (compatible fsl,mpic) with
>>>>> guest and host debug turned on.
>>>>>
>>>>> Maybe you have an idea what's going wrong.
>>>>
>>>> IIRC QEMU is missing support for large vectors, which is probably
>>>> breaking IPIs.  A recent change to Linux has it assuming it can use
>>>> large vectors when it sees fsl,mpic, as we're running out of vectors (on
>>>> p4080 MSIs collided with the arbitrarily chosen timer vector, and on
>>>> t4240 the normal internal interrupts alone go beyond 256).
>>>>
>>>> We need to get the enhancements from our internal KVM MPIC back into QEMU.
>>>
>>> Ok, so the quick fix for 1.2 would be to revert to the old compatible
>>> name. Can we leave the 4-field interrupt numbers or do we need to
>>> revert the whole patch?
>>
>> In theory you shouldn't have 4-cell interrupt numbers without fsl,mpic,
>> but I don't think it will actually break in Linux -- the extra cells
>> should just be ignored.
> 
> So I suppose the best would be to revert the whole patch and simply
> go back to the old format then, to make sure we don't introduce more
> oddness for non-Linux guests (not that I'm aware we're capable of
> running any, especially any that would use the dtb).
> 
> Once we have working large vectors in our MPIC emulation, we can
> easily put the patch into place again and generate dt's that show an
> fsl mpic.

Additionally, we should consider adding extra compatibles with the major
QEMU version in them, so that QEMU-aware target code can work around
QEMU limitations even if it's been fixed in a more recent QEMU.

I think this is what you were getting at in the e500 platform
discussion, when you pointed at the PC versioning, but it's not about
documenting semantics so much as identifying the actual implementation.

-Scott