From: Michael Kelley <redacted> Sent: Wednesday, September 1, 2021 7:34 PM

[snip]

quoted

+int netvsc_dma_map(struct hv_device *hv_dev,
+		   struct hv_netvsc_packet *packet,
+		   struct hv_page_buffer *pb)
+{
+	u32 page_count =  packet->cp_partial ?
+		packet->page_buf_cnt - packet->rmsg_pgcnt :
+		packet->page_buf_cnt;
+	dma_addr_t dma;
+	int i;
+
+	if (!hv_is_isolation_supported())
+		return 0;
+
+	packet->dma_range = kcalloc(page_count,
+				    sizeof(*packet->dma_range),
+				    GFP_KERNEL);
+	if (!packet->dma_range)
+		return -ENOMEM;
+
+	for (i = 0; i < page_count; i++) {
+		char *src = phys_to_virt((pb[i].pfn << HV_HYP_PAGE_SHIFT)
+					 + pb[i].offset);
+		u32 len = pb[i].len;
+
+		dma = dma_map_single(&hv_dev->device, src, len,
+				     DMA_TO_DEVICE);
+		if (dma_mapping_error(&hv_dev->device, dma)) {
+			kfree(packet->dma_range);
+			return -ENOMEM;
+		}
+
+		packet->dma_range[i].dma = dma;
+		packet->dma_range[i].mapping_size = len;
+		pb[i].pfn = dma >> HV_HYP_PAGE_SHIFT;
+		pb[i].offset = offset_in_hvpage(dma);
+		pb[i].len = len;
+	}

Just to confirm, this driver does *not* set the DMA min_align_mask
like storvsc does.  So after the call to dma_map_single(), the offset
in the page could be different.  That's why you are updating
the pb[i].offset value.  Alternatively, you could set the DMA
min_align_mask, which would ensure the offset is unchanged.
I'm OK with either approach, though perhaps a comment is
warranted to explain, as this is a subtle issue.

On second thought, I don't think either approach is OK.  The default
alignment in the swiotlb is 2K, and if the length of the data in the
buffer was 3K, the data could cross a page boundary in the bounce
buffer when it originally did not.  This would break the above code
which can only deal with one page at a time.  So I think the netvsc
driver also must set the DMA min_align_mask to 4K, which will
preserve the offset.

Michael