[PATCH v3 7/7] vhost: feature to set the vring endianness

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This patch brings cross-endian support to vhost when used to implement
legacy virtio devices. Since it is a relatively rare situation, the
feature availability is controlled by a kernel config option (not set
by default).

The ioctls introduced by this patch are for legacy only: virtio 1.0
devices are returned EPERM.

Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
---
 drivers/vhost/Kconfig      |   10 ++++++++
 drivers/vhost/vhost.c      |   55 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/vhost/vhost.h      |   17 +++++++++++++-
 include/uapi/linux/vhost.h |    5 ++++
 4 files changed, 86 insertions(+), 1 deletion(-)

Changes since v2:
- fixed typos in Kconfig description
- renamed vq->legacy_big_endian to vq->legacy_is_little_endian
- vq->legacy_is_little_endian reset to default in vhost_vq_reset()
- dropped VHOST_F_SET_ENDIAN_LEGACY feature
- dropped struct vhost_vring_endian from the user API (re-use
  struct vhost_vring_state instead)
- added VHOST_GET_VRING_ENDIAN_LEGACY ioctl
- introduced more helpers and stubs to avoid polluting the code with ifdefs


diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 017a1e8..0aec88c 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig
@@ -32,3 +32,13 @@ config VHOST
 	---help---
 	  This option is selected by any driver which needs to access
 	  the core of vhost.
+
+config VHOST_SET_ENDIAN_LEGACY
+	bool "Cross-endian support for host kernel accelerator"
+	default n
+	---help---
+	  This option allows vhost to support guests with a different byte
+	  ordering from host. It is disabled by default since it adds overhead
+	  and it is only needed by a few platforms (powerpc and arm).
+
+	  If unsure, say "N".
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2ee2826..3529a3c 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -199,6 +199,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	vq->call = NULL;
 	vq->log_ctx = NULL;
 	vq->memory = NULL;
+	vq->legacy_is_little_endian = virtio_legacy_is_little_endian();
 }
 
 static int vhost_worker(void *data)
@@ -630,6 +631,54 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 	return 0;
 }
 
+#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
+static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
+					  void __user *argp)
+{
+	struct vhost_vring_state s;
+
+	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
+		return -EPERM;
+
+	if (copy_from_user(&s, argp, sizeof(s)))
+		return -EFAULT;
+
+	vq->legacy_is_little_endian = !!s.num;
+	return 0;
+}
+
+static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
+					  u32 idx,
+					  void __user *argp)
+{
+	struct vhost_vring_state s = {
+		.index = idx,
+		.num = vq->legacy_is_little_endian
+	};
+
+	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
+		return -EPERM;
+
+	if (copy_to_user(argp, &s, sizeof(s)))
+		return -EFAULT;
+
+	return 0;
+}
+#else
+static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
+					  void __user *argp)
+{
+	return 0;
+}
+
+static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
+					  u32 idx,
+					  void __user *argp)
+{
+	return 0;
+}
+#endif /* CONFIG_VHOST_SET_ENDIAN_LEGACY */
+
 long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 {
 	struct file *eventfp, *filep = NULL;
@@ -806,6 +855,12 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 		} else
 			filep = eventfp;
 		break;
+	case VHOST_SET_VRING_ENDIAN_LEGACY:
+		r = vhost_set_vring_endian_legacy(vq, argp);
+		break;
+	case VHOST_GET_VRING_ENDIAN_LEGACY:
+		r = vhost_get_vring_endian_legacy(vq, idx, argp);
+		break;
 	default:
 		r = -ENOIOCTLCMD;
 	}
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 4e9a186..981ba06 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -106,6 +106,9 @@ struct vhost_virtqueue {
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
+
+	/* We need to know the device endianness with legacy virtio. */
+	bool legacy_is_little_endian;
 };
 
 struct vhost_dev {
@@ -173,11 +176,23 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
 	return vq->acked_features & (1ULL << bit);
 }
 
+#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
+static inline bool vhost_legacy_is_little_endian(struct vhost_virtqueue *vq)
+{
+	return vq->legacy_is_little_endian;
+}
+#else
+static inline bool vhost_legacy_is_little_endian(struct vhost_virtqueue *vq)
+{
+	return virtio_legacy_is_little_endian();
+}
+#endif
+
 static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
 {
 	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
 		return true;
-	return virtio_legacy_is_little_endian();
+	return vhost_legacy_is_little_endian(vq);
 }
 
 /* Memory accessors */
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index bb6a5b4..1b01a72 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -103,6 +103,11 @@ struct vhost_memory {
 /* Get accessor: reads index, writes value in num */
 #define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct vhost_vring_state)
 
+/* Set endianness for the ring (legacy virtio only) */
+/* num is 0 for big endian, other values mean little endian */
+#define VHOST_SET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
+#define VHOST_GET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
+
 /* The following ioctls use eventfd file descriptors to signal and poll
  * for events. */
 

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