vdpa_sim: fix endian-ness of config space

VDPA sim accesses config space as native endian - this is
wrong since it's a modern device and actually uses LE.

It only supports modern guests so we could punt and
just force LE, but let's use the full virtio APIs since people
tend to copy/paste code, and this is not data path anyway.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -24,6 +24,7 @@
 #include <linux/etherdevice.h>
 #include <linux/vringh.h>
 #include <linux/vdpa.h>
+#include <linux/virtio_byteorder.h>
 #include <linux/vhost_iotlb.h>
 #include <uapi/linux/virtio_config.h>
 #include <uapi/linux/virtio_net.h>
@@ -72,6 +73,23 @@ struct vdpasim {
 	u64 features;
 };
 
+/* TODO: cross-endian support */
+static inline bool vdpasim_is_little_endian(struct vdpasim *vdpasim)
+{
+	return virtio_legacy_is_little_endian() ||
+		(vdpasim->features & (1ULL << VIRTIO_F_VERSION_1));
+}
+
+static inline u16 vdpasim16_to_cpu(struct vdpasim *vdpasim, __virtio16 val)
+{
+	return __virtio16_to_cpu(vdpasim_is_little_endian(vdpasim), val);
+}
+
+static inline __virtio16 cpu_to_vdpasim16(struct vdpasim *vdpasim, u16 val)
+{
+	return __cpu_to_virtio16(vdpasim_is_little_endian(vdpasim), val);
+}
+
 static struct vdpasim *vdpasim_dev;
 
 static struct vdpasim *vdpa_to_sim(struct vdpa_device *vdpa)
@@ -306,7 +324,6 @@ static const struct vdpa_config_ops vdpasim_net_config_ops;
 
 static struct vdpasim *vdpasim_create(void)
 {
-	struct virtio_net_config *config;
 	struct vdpasim *vdpasim;
 	struct device *dev;
 	int ret = -ENOMEM;
@@ -331,10 +348,7 @@ static struct vdpasim *vdpasim_create(void)
 	if (!vdpasim->buffer)
 		goto err_iommu;
 
-	config = &vdpasim->config;
-	config->mtu = 1500;
-	config->status = VIRTIO_NET_S_LINK_UP;
-	eth_random_addr(config->mac);
+	eth_random_addr(vdpasim->config.mac);
 
 	vringh_set_iotlb(&vdpasim->vqs[0].vring, vdpasim->iommu);
 	vringh_set_iotlb(&vdpasim->vqs[1].vring, vdpasim->iommu);
@@ -448,6 +462,7 @@ static u64 vdpasim_get_features(struct vdpa_device *vdpa)
 static int vdpasim_set_features(struct vdpa_device *vdpa, u64 features)
 {
 	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
+	struct virtio_net_config *config = &vdpasim->config;
 
 	/* DMA mapping must be done by driver */
 	if (!(features & (1ULL << VIRTIO_F_ACCESS_PLATFORM)))
@@ -455,6 +470,14 @@ static int vdpasim_set_features(struct vdpa_device *vdpa, u64 features)
 
 	vdpasim->features = features & vdpasim_features;
 
+	/* We generally only know whether guest is using the legacy interface
+	 * here, so generally that's the earliest we can set config fields.
+	 * Note: We actually require VIRTIO_F_ACCESS_PLATFORM above which
+	 * implies VIRTIO_F_VERSION_1, but let's not try to be clever here.
+	 */
+
+	config->mtu = cpu_to_vdpasim16(vdpasim, 1500);
+	config->status = cpu_to_vdpasim16(vdpasim, VIRTIO_NET_S_LINK_UP);
 	return 0;
 }