Re: [PATCH v4 7/8] vhost: feature to set the vring endianness

Michael S. Tsirkin
2015-04-21T14:04:23Z

On Fri, Apr 10, 2015 at 12:19:16PM +0200, Greg Kurz wrote:
> 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 vq->is_le boolean field is added to cache the endianness to be
> used for ring accesses. It defaults to native endian, as expected
> by legacy virtio devices. When the ring gets active, we force little
> endian if the device is modern. When the ring is deactivated, we
> revert to the native endian default.
>
> If cross-endian was compiled in, a vq->user_be boolean field is added
> so that userspace may request a specific endianness. This field is
> used to override the default when activating the ring of a legacy
> device. It has no effect on modern devices.
>
> Signed-off-by: Greg Kurz <gkurz-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
> ---
> drivers/vhost/Kconfig | 10 ++++++
> drivers/vhost/vhost.c | 76 +++++++++++++++++++++++++++++++++++++++++++-
> drivers/vhost/vhost.h | 12 +++++--
> include/uapi/linux/vhost.h | 9 +++++
> 4 files changed, 103 insertions(+), 4 deletions(-)
>
> Changes since v3:
> - VHOST_SET_VRING_ENDIAN_LEGACY ioctl renamed to VHOST_SET_VRING_BIG_ENDIAN
> - ioctl API is now: 0 for le, 1 for be, other values are EINVAL
> - ioctl doesn't filter out modern devices
> - ioctl stubs return ENOIOCTLCMD
> - forbid endianness changes when vring is active
> - logic now handled with vq->is_le and vq->user_be according to device
> start/stop as suggested by Michael
>
> 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

I'd prefer namin this VHOST_CROSS_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).

and is only useful on a few platforms (powerpc and arm).

"it" seems to refer to "overhead", which is rarely needed.
needed is a bit too strong, you can always e.g. run virtio
in userspace.

> +
> + If unsure, say "N".
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 2ee2826..3eb756b 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -199,6 +199,10 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> vq->call = NULL;
> vq->log_ctx = NULL;
> vq->memory = NULL;
> + vq->is_le = virtio_legacy_is_little_endian();
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> + vq->user_be = !vq->is_le;
> +#endif

add a wrapper for this too?

> }
>
> static int vhost_worker(void *data)
> @@ -630,6 +634,53 @@ 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_big_endian(struct vhost_virtqueue *vq,
> + int __user *argp)
> +{
> + struct vhost_vring_state s;
> +
> + if (vq->private_data)
> + return -EBUSY;
> +
> + if (copy_from_user(&s, argp, sizeof(s)))
> + return -EFAULT;
> +
> + if (s.num && s.num != 1)

s.num & ~0x1

> + return -EINVAL;
> +
> + vq->user_be = s.num;
> +
> + return 0;
> +}
> +
> +static long vhost_get_vring_big_endian(struct vhost_virtqueue *vq, u32 idx,
> + int __user *argp)
> +{
> + struct vhost_vring_state s = {
> + .index = idx,
> + .num = vq->user_be
> + };
> +
> + if (copy_to_user(argp, &s, sizeof(s)))
> + return -EFAULT;
> +
> + return 0;
> +}
> +#else
> +static long vhost_set_vring_big_endian(struct vhost_virtqueue *vq,
> + int __user *argp)
> +{
> + return -ENOIOCTLCMD;
> +}
> +
> +static long vhost_get_vring_big_endian(struct vhost_virtqueue *vq, u32 idx,
> + int __user *argp)
> +{
> + return -ENOIOCTLCMD;
> +}
> +#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 +857,12 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> } else
> filep = eventfp;
> break;
> + case VHOST_SET_VRING_BIG_ENDIAN:
> + r = vhost_set_vring_big_endian(vq, argp);
> + break;
> + case VHOST_GET_VRING_BIG_ENDIAN:
> + r = vhost_get_vring_big_endian(vq, idx, argp);
> + break;
> default:
> r = -ENOIOCTLCMD;
> }
> @@ -1040,12 +1097,29 @@ static int vhost_update_avail_event(struct vhost_virtqueue *vq, u16 avail_event)
> return 0;
> }
>
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> +{
> + vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
> +}
> +#else
> +static void vhost_init_is_le(struct vhost_virtqueue *vq)
> +{
> + if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> + vq->is_le = true;
> +}
> +#endif
> +

I'd prefer localizing ifdefery somewhere near top of file.

> int vhost_init_used(struct vhost_virtqueue *vq)
> {
> __virtio16 last_used_idx;
> int r;
> - if (!vq->private_data)
> + if (!vq->private_data) {
> + vq->is_le = virtio_legacy_is_little_endian();
> 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 4e9a186..04b2add 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -106,6 +106,14 @@ struct vhost_virtqueue {
> /* Log write descriptors */
> void __user *log_base;
> struct vhost_log *log;
> +
> + /* Ring endianness. Defaults to legacy native endianness.
> + * Set to true when starting a modern virtio device. */
> + bool is_le;
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> + /* Ring endianness requested by userspace for cross-endian support. */
> + bool user_be;
> +#endif
> };
>
> struct vhost_dev {
> @@ -175,9 +183,7 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
>
> 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 vq->is_le;
> }
>
> /* Memory accessors */
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index bb6a5b4..5cdebbc 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -103,6 +103,15 @@ 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 the vring byte order in num. This is a legacy only API that is simply
> + * ignored when VIRTIO_F_VERSION_1 is set.
> + * 0 to set to little-endian
> + * 1 to set to big-endian

How about defines for these?

> + * other values return EINVAL.
> + */
> +#define VHOST_SET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
> +#define VHOST_GET_VRING_BIG_ENDIAN _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
> +
> /* The following ioctls use eventfd file descriptors to signal and poll
> * for events. */
>