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

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Date:   Mon, 21 Jan 2019 04:51:57 +0000
From:   Peng Fan <peng.fan@....com>
To:     "mst@...hat.com" <mst@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "sstabellini@...nel.org" <sstabellini@...nel.org>,
        "hch@...radead.org" <hch@...radead.org>
CC:     "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>,
        Peng Fan <peng.fan@....com>
Subject: [RFC] virtio_ring: check dma_mem for xen_domain

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.

With this patch, vring_use_dma_api will return false, and
vring_map_one_sg will return sg_phys(sg) which is the correct phys
address in the predefined memory region.
vring_map_one_sg -> vring_use_dma_api
                 -> sg_phys(sg)

Signed-off-by: Peng Fan <peng.fan@....com>
---
 drivers/virtio/virtio_ring.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cd7e755484e3..8993d7cb3592 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -248,6 +248,8 @@ static inline bool virtqueue_use_indirect(struct virtqueue *_vq,
 
 static bool vring_use_dma_api(struct virtio_device *vdev)
 {
+	struct device *dma_dev = vdev->dev.parent;
+
 	if (!virtio_has_iommu_quirk(vdev))
 		return true;
 
@@ -260,7 +262,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
 	 * the DMA API if we're a Xen guest, which at least allows
 	 * all of the sensible Xen configurations to work correctly.
 	 */
-	if (xen_domain())
+	if (xen_domain() && !dma_dev->dma_mem)
 		return true;
 
 	return false;
-- 
2.14.1