swiotlb: rewrite comment explaining why the source is preserved on DMA_FROM_DEVICE

Rewrite the comment explaining why swiotlb copies the original buffer to
the TLB buffer before initiating DMA *from* the device, i.e. before the
device DMAs into the TLB buffer.  The existing comment's argument that
preserving the original data can prevent a kernel memory leak is bogus.

If the driver that triggered the mapping _knows_ that the device will
overwrite the entire mapping, or the driver will consume only the written
parts, then copying from the original memory is completely pointless.

If neither of the above holds true, then copying from the original adds
value only if preserving the data is necessary for functional
correctness, or the driver explicitly initialized the original memory.
If the driver didn't initialize the memory, then copying the original
buffer to the TLB buffer simply changes what kernel data is leaked to
user space.

Writing the entire TLB buffer _does_ prevent leaking stale TLB buffer
data from a previous bounce, but that can be achieved by simply zeroing
the TLB buffer when grabbing a slot.

The real reason swiotlb ended up initializing the TLB buffer with the
original buffer is that it's necessary to make swiotlb operate as
transparently as possible, i.e. to behave as closely as possible to
hardware, and to avoid corrupting the original buffer, e.g. if the driver
knows the device will do partial writes and is relying on the unwritten
data to be preserved.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/all/ZN5elYQ5szQndN8n@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

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,
+	 * i.e. swiotlb must not corrupt memory by clobbering unwritten bytes.
 	 */
 	swiotlb_bounce(dev, tlb_addr, mapping_size, DMA_TO_DEVICE);
 	return tlb_addr;