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

[Ard Biesheuvel]

On 13 January 2017 at 11:47, Robin Murphy <robin.murphy at arm.com> wrote:
> On 13/01/17 11:25, Ard Biesheuvel wrote:
>> On 13 January 2017 at 11:03, Robin Murphy <robin.murphy at 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?