On Mon, Mar 7, 2022 at 2:36 PM Robin Murphy <robin.murphy@xxxxxxx> wrote: > > On 2022-03-07 12:17, Gilad Ben-Yossef wrote: > > 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; > > It's OK to have multiple mappings that are *all* DMA_TO_DEVICE, which > looks to be the case that this check was intended to allow. However I > think you're right that it should still actually check for conflicting > directions between the new entry and any existing ones, otherwise it > ends up a bit too lenient. > > Cheers, > Robin. I understand what you are saying about why checking for conflicting directions may be a good thing, but given that the code is as it is right now, how are we seeing the warning for two mapping that one of them is DMA_TO_DEVICE? Gilad -- Gilad Ben-Yossef Chief Coffee Drinker values of β will give rise to dom!
