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SubjectRe: [PATCH] vhost: move is_le setup to the backend
On Mon, Jan 11, 2016 at 03:39:38PM +0100, Greg Kurz wrote:
> The vq->is_le field is used to fix endianness when accessing the vring via
> the cpu_to_vhost16() and vhost16_to_cpu() helpers in the following cases:
>
> 1) host is big endian and device is modern virtio
>
> 2) host has cross-endian support and device is legacy virtio with a different
> endianness than the host
>
> Both cases rely on the VHOST_SET_FEATURES ioctl, but 2) also needs the
> VHOST_SET_VRING_ENDIAN ioctl to be called by userspace. Since vq->is_le
> is only needed when the backend is active, it was decided to set it at
> backend start.
>
> This is currently done in vhost_init_used()->vhost_init_is_le() but it
> obfuscates the core vhost code. This patch moves the is_le setup to a
> dedicated function that is called from the backend code.
>
> Note vhost_net is the only backend that can pass vq->private_data == NULL to
> vhost_init_used(), hence the "if (sock)" branch.
>
> No behaviour change.
>
> Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>
> Hi Michael,
>
> This is a cleanup patch I had posted last year:
>
> http://patchwork.ozlabs.org/patch/538277/
>
> I am just reposting with Cornelia's R-b.
>
> Cheers.
>
> --
> Greg
>
> drivers/vhost/net.c | 6 ++++++
> drivers/vhost/scsi.c | 3 +++
> drivers/vhost/test.c | 2 ++
> drivers/vhost/vhost.c | 12 +++++++-----
> drivers/vhost/vhost.h | 1 +
> 5 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9eda69e40678..d6319cb2664c 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -917,6 +917,12 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>
> vhost_net_disable_vq(n, vq);
> vq->private_data = sock;
> +
> + if (sock)
> + vhost_set_is_le(vq);
> + else
> + vq->is_le = virtio_legacy_is_little_endian();
> +
> r = vhost_init_used(vq);
> if (r)
> goto err_used;

This part is kind of ugly. I think it's cleaner is the generic code.
How about we teach vhost_set_is_le to test vq->private_data and DTRT?

> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index 29cfc57d496e..1f4f405ebba8 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_set_is_le(vq);
> +
> vhost_init_used(vq);
> mutex_unlock(&vq->mutex);
> }
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index f2882ac98726..b1c7df502211 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_set_is_le(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 ad2146a9ab2d..e688eb801d43 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -113,6 +113,12 @@ static void vhost_init_is_le(struct vhost_virtqueue *vq)
> }
> #endif /* CONFIG_VHOST_CROSS_ENDIAN_LEGACY */
>
> +void vhost_set_is_le(struct vhost_virtqueue *vq)
> +{
> + vhost_init_is_le(vq);
> +}
> +EXPORT_SYMBOL_GPL(vhost_set_is_le);
> +
> static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
> poll_table *pt)
> {

Why do we need this wrapper? Why not just export vhost_init_is_le?
Or rename vhost_init_is_le->vhost_set_is_le if you prefer.

> @@ -1156,12 +1162,8 @@ int vhost_init_used(struct vhost_virtqueue *vq)
> {
> __virtio16 last_used_idx;
> int r;
> - if (!vq->private_data) {
> - vq->is_le = virtio_legacy_is_little_endian();
> + if (!vq->private_data)
> return 0;
> - }
> -
> - vhost_init_is_le(vq);
>
> r = vhost_update_used_flags(vq);
> if (r)
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index d3f767448a72..af5d33797937 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_set_is_le(struct vhost_virtqueue *vq);
>
> #define vq_err(vq, fmt, ...) do { \
> pr_debug(pr_fmt(fmt), ##__VA_ARGS__); \

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