Re: [RFC] virtio_ring: check dma_mem for xen_domain

[hch@xxxxxxxxxxxxx]

On Mon, Jan 21, 2019 at 04:51:57AM +0000, Peng Fan wrote:
> on i.MX8QM, M4_1 is communicating with DomU using rpmsg with a fixed
> address as the dma mem buffer which is predefined.
> 
> Without this patch, the flow is:
> vring_map_one_sg -> vring_use_dma_api
>                  -> dma_map_page
> 		       -> __swiotlb_map_page
> 		                ->swiotlb_map_page
> 				->__dma_map_area(phys_to_virt(dma_to_phys(dev, dev_addr)), size, dir);
> However we are using per device dma area for rpmsg, phys_to_virt
> could not return a correct virtual address for virtual address in
> vmalloc area. Then kernel panic.

And that is the right thing to do.  You must not call dma_map_* on
memory that was allocated from dma_alloc_*.

We actually have another thread which appears to be for this same issue.