[RFC] Kernel panic down to swiotlb when doing insmod a simple driver

[robin.murphy@arm.com (Robin Murphy)]

On 13/01/17 11:54, Ard Biesheuvel wrote:
> On 13 January 2017 at 11:52, Robin Murphy <robin.murphy@arm.com> wrote:
>> On 13/01/17 11:49, Ard Biesheuvel wrote:
>>> On 13 January 2017 at 11:47, Robin Murphy <robin.murphy@arm.com> wrote:
>>>> On 13/01/17 11:25, Ard Biesheuvel wrote:
>>>>> On 13 January 2017 at 11:03, Robin Murphy <robin.murphy@arm.com> wrote:
>>>>>> On 13/01/17 10:00, Shawn Lin wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> Sorry for sending this RFC for help as I couldn't find some useful hint
>>>>>>> to slove my issue by git-log the swiotlb commit from kernel v4.4 to
>>>>>>> v4.9 and I'm also not familar with these stuff. So could you kindly
>>>>>>> point me to the right direction to debug it? Thanks. :)
>>>>>>>
>>>>>>> --------------------------------------
>>>>>>> We just have a very simple wifi driver *built as ko module* which only
>>>>>>> have a probe function to do the basic init work and call SDIO API to
>>>>>>> transfer some bytes.
>>>>>>>
>>>>>>> Env: kernel 4.4 stable tree, ARM64(rk3399)
>>>>>>>
>>>>>>> Two cases are included:
>>>>>>
>>>>>> And they are both wrong :)
>>>>>>
>>>>>>> The crash case:
>>>>>>>
>>>>>>> u8 __aligned(32) buf[PAGE_SIZE]; //global here in ko driver file
>>>>>>
>>>>>> It is only valid to do DMA from linear map addresses - I'm not sure if
>>>>>> the modules area was in the linear map before, but either way it
>>>>>> probably isn't now (Ard, Mark?). Either way, I don't believe static data
>>>>>> honours ARCH_DMA_MINALIGN in general, so it's still highly inadvisable.
>>>>>>
>>>>>
>>>>> The __aligned() modifier should work fine: the alignment is propagated
>>>>> to the ELF section alignment, which in turn is honoured by the module
>>>>> loader. The problem is that '32' is too low for non-coherent DMA to be
>>>>> safe. In general, alignments up to 4 KB should work everywhere.
>>>>
>>>> Does that alignment also implicitly apply to the size, though? In other
>>>> words, given:
>>>>
>>>> static int X
>>>> static int __aligned(32) Y;
>>>> static int Z;
>>>>
>>>> is it guaranteed that if, say, X gets placed at .data + 0, so Y goes to
>>>> .data + 32, then Z *cannot* be placed at .data + 36?
>>>>
>>>
>>> I'm not sure if I understand the question: why would it be incorrect
>>> for Z to be placed at .data + 36?
>>
>> Because they'd then wind up in the same cache line, so non-coherent DMA
>> to Y will result in concurrent CPU updates to Z being lost/corrupted.
>> ARCH_DMA_MINALIGN isn't about alignemnt per se, it's about keeping
>> things in distinct cache writeback granules.
>>
> 
> I understand that. But the original code did
> 
>  u8 __aligned(32) buf[PAGE_SIZE]; //global here in ko driver file
> 
> so there the size is guaranteed to be a multiple of the CWG
> 
> So to answer your question: no, the compiler will not round up the
> size of the allocation to the alignment, it will only align the start.

Right. I was deliberately ignoring the "you happen to get away with it
in this case because..." part because I think the "DMA to static
data/stack is not safe in general" message is important :)

Thanks for the clarification.

Robin.