On Mon, Jun 14, 2021 at 10:04:06PM +0800, Tianyu Lan wrote:

The pages in the hv_page_buffer array here are in the kernel linear 
mapping. The packet sent to host will contain an array which contains 
transaction data. In the isolation VM, data in the these pages needs to be 
copied to bounce buffer and so call dma_map_single() here to map these data 
pages with bounce buffer. The vmbus has ring buffer where the send/receive 
packets are copied to/from. The ring buffer has been remapped to the extra 
space above shared gpa boundary/vTom during probing Netvsc driver and so 
not call dma map function for vmbus ring
buffer.

So why do we have all that PFN magic instead of using struct page or
the usual kernel I/O buffers that contain a page pointer?