Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for netvsc driver

[Tianyu Lan]

On 6/14/2021 11:33 PM, Christoph Hellwig wrote:
> 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?

These PFNs originally is part of Hyper-V protocol data and will be sent
to host. Host accepts these GFN and copy data from/to guest memory. The 
translation from va to pa is done by caller that populates the 
hv_page_buffer array. I will try calling dma map function before 
populating struct hv_page_buffer and this can avoid redundant 
translation between PA and VA.