Re: [RFC PATCH 1/4] vhost: add VHOST_VRING_F_BYTESWAP flag

Michael S. Tsirkin
2014-11-03T16:29:06Z

On Wed, Oct 29, 2014 at 09:38:42AM +0100, Cédric Le Goater wrote:
> The VHOST_VRING_F_BYTESWAP flag will be used by the host to byteswap
> the vring data when the guest and the host have a different endian
> order.
>
> Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>

I don't think it's a good API.
You should ask for specific header format, not for a swap.

> ---
> drivers/vhost/vhost.c | 5 ++++-
> drivers/vhost/vhost.h | 1 +
> include/uapi/linux/vhost.h | 3 +++
> 3 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index c90f4374442a..72c21b790ba3 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->byteswap = 0;
> }
>
> static int vhost_worker(void *data)
> @@ -701,7 +702,8 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> r = -EFAULT;
> break;
> }
> - if (a.flags & ~(0x1 << VHOST_VRING_F_LOG)) {
> + if (a.flags & ~(0x1 << VHOST_VRING_F_LOG |
> + 0x1 << VHOST_VRING_F_BYTESWAP)) {
> r = -EOPNOTSUPP;
> break;
> }
> @@ -747,6 +749,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> vq->avail = (void __user *)(unsigned long)a.avail_user_addr;
> vq->log_addr = a.log_guest_addr;
> vq->used = (void __user *)(unsigned long)a.used_user_addr;
> + vq->byteswap = !!(a.flags & (0x1 << VHOST_VRING_F_BYTESWAP));
> break;
> case VHOST_SET_VRING_KICK:
> if (copy_from_user(&f, argp, sizeof f)) {
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 3eda654b8f5a..ab25b7d0720d 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -110,6 +110,7 @@ struct vhost_virtqueue {
> /* Log write descriptors */
> void __user *log_base;
> struct vhost_log *log;
> + bool byteswap;
> };
>
> struct vhost_dev {
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index bb6a5b4cb3c5..6a8c2b325c44 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -34,6 +34,9 @@ struct vhost_vring_addr {
> /* Flag values: */
> /* Whether log address is valid. If set enables logging. */
> #define VHOST_VRING_F_LOG 0
> + /* Whether vring memory accesses should be byte-swapped.
> + * required when the guest has a different endianness */
> +#define VHOST_VRING_F_BYTESWAP 1
>
> /* Start of array of descriptors (virtually contiguous) */
> __u64 desc_user_addr;
> --
> 1.7.10.4