On 2022-03-07 13:03, Robin Murphy wrote:
On 2022-03-07 12:47, Gilad Ben-Yossef wrote:
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?
Because it's the second one that isn't. The warning is triggered by
adding the DMA_FROM_DEVICE entry, which *is* checked, and finds the
DMA_TO_DEVICE entry already present. What's not great is that if those
two mappings happened to be made in the opposite order then it would be
missed entirely.
Urgh, no, sorry, that's some imaginary conflation of the cacheline radix
tree with the entry hash bucket...
What's actually happened here is that I've failed to read the log
properly and they're both DMA_FROM_DEVICE. But the potential problem of
mixed-direction mappings being missed does still stand in general.
Robin.
