Re: [RFC PATCH 3/4] vhost: byteswap virtqueue attributes

Cornelia Huck
2014-11-03T16:02:33Z

On Wed, 29 Oct 2014 09:38:44 +0100
Cédric Le Goater <clg@fr.ibm.com> wrote:

> The virtqueue structure shares a few attributes with the guest OS
> which need to be byteswapped when the endian order of the host is
> different.
>
> This patch uses the vq->byteswap attribute to decide whether to
> byteswap or not data being accessed in the guest memory.
>
> Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
> ---
> drivers/vhost/vhost.c | 52 +++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 40 insertions(+), 12 deletions(-)
>

> +static int __copyhead_to_user(struct vhost_virtqueue *vq,
> + struct vring_used_elem *heads,
> + struct vring_used_elem __user *used,
> + unsigned count)

__copy_used_elems_to_user() ?

> +{
> + int i;
> +
> + for (i = 0; i < count; i++) {
> + if (__vq_put_user(vq, heads[i].id, &used[i].id)) {
> + vq_err(vq, "Failed to write used id");
> + return -EFAULT;
> + }
> + if (__vq_put_user(vq, heads[i].len, &used[i].len)) {
> + vq_err(vq, "Failed to write used len");
> + return -EFAULT;
> + }
> + }

Is there a number of elements where it would be more efficient to
byteswap the used elements first and then do __copy_to_user() in one
go? Depends on the backend, I guess.

> + return 0;
> +}
> +