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

[Stefano Stabellini]

On Mon, 21 Jan 2019, 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.
> 
> 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@xxxxxxx>
> ---
>  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;

I can see you spotted a real issue, but this is not the right fix. We
just need something a bit more flexible than xen_domain(): there are
many kinds of Xen domains on different architectures, we basically want
to enable this (return true from vring_use_dma_api) only when the xen
swiotlb is meant to be used. Does the appended patch fix the issue you
have?

---

xen: introduce xen_vring_use_dma

From: Stefano Stabellini <stefanos@xxxxxxxxxx>

Export xen_swiotlb on arm and arm64.

Use xen_swiotlb to determine when vring should use dma APIs to map the
ring: when xen_swiotlb is enabled the dma API is required. When it is
disabled, it is not required.

Reported-by: Peng Fan <peng.fan@xxxxxxx>
Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>

diff --git a/arch/arm/include/asm/xen/swiotlb-xen.h b/arch/arm/include/asm/xen/swiotlb-xen.h
new file mode 100644
index 0000000..455ade5
--- /dev/null
+++ b/arch/arm/include/asm/xen/swiotlb-xen.h
@@ -0,0 +1 @@
+#include <xen/arm/swiotlb-xen.h>
diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
index cb44aa2..8592863 100644
--- a/arch/arm/xen/mm.c
+++ b/arch/arm/xen/mm.c
@@ -21,6 +21,8 @@
 #include <asm/xen/hypercall.h>
 #include <asm/xen/interface.h>
 
+int xen_swiotlb __read_mostly;
+
 unsigned long xen_get_swiotlb_free_pages(unsigned int order)
 {
 	struct memblock_region *reg;
@@ -189,6 +191,7 @@ int __init xen_mm_init(void)
 	struct gnttab_cache_flush cflush;
 	if (!xen_initial_domain())
 		return 0;
+	xen_swiotlb = 1;
 	xen_swiotlb_init(1, false);
 	xen_dma_ops = &xen_swiotlb_dma_ops;
 
diff --git a/arch/arm64/include/asm/xen/swiotlb-xen.h b/arch/arm64/include/asm/xen/swiotlb-xen.h
new file mode 100644
index 0000000..455ade5
--- /dev/null
+++ b/arch/arm64/include/asm/xen/swiotlb-xen.h
@@ -0,0 +1 @@
+#include <xen/arm/swiotlb-xen.h>
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cd7e755..bf8badc 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -260,7 +260,7 @@ 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_vring_use_dma())
 		return true;
 
 	return false;
diff --git a/include/xen/arm/swiotlb-xen.h b/include/xen/arm/swiotlb-xen.h
new file mode 100644
index 0000000..2aac7c4
--- /dev/null
+++ b/include/xen/arm/swiotlb-xen.h
@@ -0,0 +1,10 @@
+#ifndef _ASM_ARM_XEN_SWIOTLB_XEN_H
+#define _ASM_ARM_XEN_SWIOTLB_XEN_H
+
+#ifdef CONFIG_SWIOTLB_XEN
+extern int xen_swiotlb;
+#else
+#define xen_swiotlb (0)
+#endif
+
+#endif
diff --git a/include/xen/xen.h b/include/xen/xen.h
index 0e21567..74a536d 100644
--- a/include/xen/xen.h
+++ b/include/xen/xen.h
@@ -46,4 +46,10 @@ enum xen_domain_type {
 bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 		const struct bio_vec *vec2);
 
+#include <asm/xen/swiotlb-xen.h>
+static inline int xen_vring_use_dma(void)
+{
+	return !!xen_swiotlb;
+}
+
 #endif	/* _XEN_XEN_H */