On Mon, Mar 7, 2022 at 1:14 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> The "overlap" is in the sense of having more than one mapping within the
> same cacheline:
>
> [ 142.458120] DMA-API: add_dma_entry start P=ba79f200 N=ba79f
> D=ba79f200 L=10 DMA_FROM_DEVICE attrs=0
> [ 142.458156] DMA-API: add_dma_entry start P=445dc010 N=445dc
> D=445dc010 L=10 DMA_TO_DEVICE attrs=0
> [ 142.458178] sun8i-ss 1c15000.crypto: SRC 0/1/1 445dc000 len=16 bi=0
> [ 142.458215] sun8i-ss 1c15000.crypto: DST 0/1/1 ba79f200 len=16 bi=0
> [ 142.458234] DMA-API: add_dma_entry start P=ba79f210 N=ba79f
> D=ba79f210 L=10 DMA_FROM_DEVICE attrs=0
>
> This actually illustrates exactly the reason why this is unsupportable.
> ba79f200 is mapped for DMA_FROM_DEVICE, therefore subsequently mapping
> ba79f210 for DMA_TO_DEVICE may cause the cacheline covering the range
> ba79f200-ba79f23f to be written back over the top of data that the
> device has already started to write to memory. Hello data corruption.
>
> Separate DMA mappings should be from separate memory allocations,
> respecting ARCH_DMA_MINALIGN.
hmm... I know I'm missing something here, but how does this align with
the following from active_cacheline_insert() in kernel/dma/debug.c ?
/* If the device is not writing memory then we don't have any
* concerns about the cpu consuming stale data. This mitigates
* legitimate usages of overlapping mappings.
*/
if (entry->direction == DMA_TO_DEVICE)
return 0;
Thanks,
Gilad
--
Gilad Ben-Yossef
Chief Coffee Drinker
values of β will give rise to dom!
