Messages in this thread | | | Date | Fri, 20 Oct 2023 10:51:12 +0100 | Subject | Re: [PATCH] swiotlb: Rewrite comment explaining why the source is preserved on DMA_FROM_DEVICE | From | Robin Murphy <> |
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On 2023-10-20 00:25, Sean Christopherson wrote: > On Wed, Oct 18, 2023, Robin Murphy wrote: >> On 2023-10-18 18:34, Sean Christopherson wrote: >>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c >>> index 01637677736f..e071415a75dc 100644 >>> --- a/kernel/dma/swiotlb.c >>> +++ b/kernel/dma/swiotlb.c >>> @@ -1296,11 +1296,13 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr, >>> pool->slots[index + i].orig_addr = slot_addr(orig_addr, i); >>> tlb_addr = slot_addr(pool->start, index) + offset; >>> /* >>> - * When dir == DMA_FROM_DEVICE we could omit the copy from the orig >>> - * to the tlb buffer, if we knew for sure the device will >>> - * overwrite the entire current content. But we don't. Thus >>> - * unconditional bounce may prevent leaking swiotlb content (i.e. >>> - * kernel memory) to user-space. >>> + * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy >>> + * the original buffer to the TLB buffer before initiating DMA in order >>> + * to preserve the original's data if the device does a partial write, >>> + * i.e. if the device doesn't overwrite the entire buffer. Preserving >>> + * the original data, even if it's garbage, is necessary to match >>> + * hardware behavior (use of swiotlb is supposed to be transparent) and >> >> Super-nit: I think that last "and" is superfluous (i.e. unwritten memory not >> magically corrupting itself *is* the aforementioned hardware behaviour). > > Ah yeah, agreed. How about this? > > /* > * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy > * the original buffer to the TLB buffer before initiating DMA in order > * to preserve the original's data if the device does a partial write, > * i.e. if the device doesn't overwrite the entire buffer. Preserving > * the original data, even if it's garbage, is necessary to match > * hardware behavior. Use of swiotlb is supposed to be transparent, > * i.e. swiotlb must not corrupt memory by clobbering unwritten bytes. > */
Nice, that reads even more clearly IMO.
Cheers, Robin.
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