linux-kernel - Re: [RFC] virtio_ring: check dma_mem for xen

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Message-ID: <20190121082830.GC12420@infradead.org>
Date:   Mon, 21 Jan 2019 00:28:30 -0800
From:   "hch@...radead.org" <hch@...radead.org>
To:     Peng Fan <peng.fan@....com>
Cc:     "mst@...hat.com" <mst@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "sstabellini@...nel.org" <sstabellini@...nel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-remoteproc@...r.kernel.org" <linux-remoteproc@...r.kernel.org>
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_*.

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