Messages in this thread |  | | Subject | Re: igb driver can cause cache invalidation of non-owned memory? | From | Nikita Yushchenko <> | Date | Mon, 10 Oct 2016 15:27:47 +0300 |
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>> Hmm... I'm not about device writing to memory. > > This absolutely is about whether the device wrote into the > area or not.
Not only.
>> Sequence in igb driver is: >> >> dma_map(full_page) >> <device writes here> >> sync_to_cpu(half_page); >> skb_add_rx_frag(skb, half_page); >> napi_gro_receive(skb); >> ... >> dma_unmap(full_page) >> >> What I'm concerned about is - same area is first passed up to network >> stack, and _later_ dma_unmap()ed. Is this indeed safe? > > dma_unmap() should never write anything unless the device has > meanwhile written to that chunk of memory.
dma_unmap() for DMA_FROM_DEVICE never writes whatever to memory, regardless of what device did.
dma_unmap() for DMA_FROM_DEVICE ensures that data written to memory by device (if any) is visible to CPU. Cache may contain stale data for that memory region. To drop that from cache, dma_unmap() for DMA_FROM_DEVICE does cache invalidation.
static void arm_dma_unmap_page(struct device *dev, dma_addr_t handle, size_t size, enum dma_data_direction dir, unsigned long attrs) { if ((attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0) __dma_page_dev_to_cpu(pfn_to_page(dma_to_pfn(dev, handle)), handle & ~PAGE_MASK, size, dir); }
static void __dma_page_dev_to_cpu(struct page *page, unsigned long off, size_t size, enum dma_data_direction dir) { ... if (dir != DMA_TO_DEVICE) { outer_inv_range(paddr, paddr + size);
dma_cache_maint_page(page, off, size, dir, dmac_unmap_area); } ... }
> If the device made no intervening writes into the area, dma_unmap() > should not cause any data to be written to that area, period.
I'm not about writing.
I'm about just the opposite - dropping not-written data from cache.
- napi_gro_receive(skb) passes area to upper layers of the network stack, - something in those layers - perhaps packet mangling or such - writes to the area, - this write enters cache but does not end into memory immediately, - at this moment, igb does dma_unmap(), - write that was in cache but not yet in memory gets lost.
> In your example above, consider the case where the device never > writes into the memory area after sync_to_cpu(). In that case > there is nothing that dma_unmap() can possibly write. > All the data has been synced
Non-synced data is write done by CPU executing upper layers of network stack,
Nikita
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