On Wed, 10 Feb 2016 13:48:09 +0200
"Michael S. Tsirkin" [off-list ref] wrote:

On Wed, Jan 13, 2016 at 06:09:47PM +0100, Greg Kurz wrote:

quoted

The way vring endianness is being handled currently obfuscates
the code in vhost_init_used().

This patch tries to fix that by doing the following:
- move the the code that adjusts endianness to a dedicated helper
- export this helper so that backends explicitely call it

No behaviour change.

Signed-off-by: Greg Kurz <redacted>
---
 drivers/vhost/net.c   |    3 +++
 drivers/vhost/scsi.c  |    3 +++
 drivers/vhost/test.c  |    2 ++
 drivers/vhost/vhost.c |   16 +++++++++++-----
 drivers/vhost/vhost.h |    1 +
 5 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 9eda69e40678..df01c939cd00 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -917,6 +917,9 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 
 		vhost_net_disable_vq(n, vq);
 		vq->private_data = sock;
+
+		vhost_adjust_vring_endian(vq);
+
 		r = vhost_init_used(vq);
 		if (r)
 			goto err_used;  

This is in fact a bug in existing code: if vhost_init_used
fails, it preferably should not have side-effects.
It's best to update it last thing.

I'm afraid we can't because the following path needs the
vring endianness:

vhost_init_used()->vhost_update_used_flags()->cpu_to_vhost16()

But you are right, there is a bug: we should rollback if vhost_init_used()
fails. Something like below:

 err_used:
        vq->private_data = oldsock;
        vhost_net_enable_vq(n, vq);
+       vhost_adjust_vring_endian(vq);
        if (ubufs)
                vhost_net_ubuf_put_wait_and_free(ubufs);
 err_ubufs:

quoted

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 29cfc57d496e..5a8363bfcb74 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c

@@ -1274,6 +1274,9 @@ vhost_scsi_set_endpoint(struct vhost_scsi *vs,
 			vq = &vs->vqs[i].vq;
 			mutex_lock(&vq->mutex);
 			vq->private_data = vs_tpg;
+
+			vhost_adjust_vring_endian(vq);
+
 			vhost_init_used(vq);
 			mutex_unlock(&vq->mutex);
 		}

diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
index f2882ac98726..75e3e0e9f5a8 100644
--- a/drivers/vhost/test.c
+++ b/drivers/vhost/test.c

@@ -196,6 +196,8 @@ static long vhost_test_run(struct vhost_test *n, int test)
 		oldpriv = vq->private_data;
 		vq->private_data = priv;
 
+		vhost_adjust_vring_endian(vq);
+
 		r = vhost_init_used(&n->vqs[index]);
 
 		mutex_unlock(&vq->mutex);

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index e02e06755ab7..b0a00340309e 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -123,6 +123,15 @@ static void vhost_disable_is_le(struct vhost_virtqueue *vq)
 	vq->is_le = virtio_legacy_is_little_endian();
 }
 
+void vhost_adjust_vring_endian(struct vhost_virtqueue *vq)
+{
+	if (!vq->private_data)
+		vhost_disable_is_le(vq);
+	else
+		vhost_enable_is_le(vq);
+}
+EXPORT_SYMBOL_GPL(vhost_adjust_vring_endian);
+
 static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
 			    poll_table *pt)
 {  

I'd prefer "vhost_update_is_le" here. "endian" might also mean
"user_be". But see below pls.

Ok, I will rename if this patch survives the review.

quoted

@@ -1166,12 +1175,9 @@ int vhost_init_used(struct vhost_virtqueue *vq)
 {
 	__virtio16 last_used_idx;
 	int r;
-	if (!vq->private_data) {
-		vhost_disable_is_le(vq);
-		return 0;
-	}
 
-	vhost_enable_is_le(vq);
+	if (!vq->private_data)
+		return 0;
 
 	r = vhost_update_used_flags(vq);
 	if (r)  

Looking at how callers use this, maybe we should just rename init_used
to vhost_vq_init_access. The _used suffix was a hint that we
access the vq used ring. But maybe what callers care about is
that it must be called after access_ok.

And so we would keep the current logic where it is up to the core
code to update is_le... that is basically getting rid of this patch :)

So IIUC you're asking for:
- fix the side-effect in vhost_init_used()
- rename vhost_init_used() to vhost_vq_init_access()

quoted

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index d3f767448a72..88d86f45f756 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h

@@ -162,6 +162,7 @@ bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
 
 int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
 		    unsigned int log_num, u64 len);
+void vhost_adjust_vring_endian(struct vhost_virtqueue *vq);
 
 #define vq_err(vq, fmt, ...) do {                                  \
 		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \