Re: [Xen-devel] [RFC] virtio_ring: check dma_mem for xen_domain

[Peng Fan <peng.fan@xxxxxxx>]

Hi

> -----Original Message-----
> From: hch@xxxxxxxxxxxxx [mailto:hch@xxxxxxxxxxxxx]
> Sent: 2019年1月21日 16:29
> To: Peng Fan <peng.fan@xxxxxxx>
> Cc: mst@xxxxxxxxxx; jasowang@xxxxxxxxxx; sstabellini@xxxxxxxxxx;
> hch@xxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx;
> xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> linux-remoteproc@xxxxxxxxxxxxxxx
> Subject: Re: [RFC] virtio_ring: check dma_mem for xen_domain
> 
> 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_*.

Understand. But the current code is that vring_use_dma_api will always return
true, if the current OS is a xen VM.

Actually it needs to return false for my case, then we could use sg_phys(sg)
to get the correct physical address.

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

You mean https://patchwork.kernel.org/patch/10742923/ ?

You suggest use cma there, but vring_use_dma_api will still return true if the 
OS
is running on xen. Then vring_map_one_sg will still runs into __dma_map_area.

In my case, just need vring_use_dma_api to return false and use sg_phys(sg) to
get the correct physical address, whether per dma reserved area or per device 
cma.

Thanks,
Peng.
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