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

[Michael S. Tsirkin]

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@xxxxxxxxxxxxxxxxxx>
> ---
>  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. */
>  
--
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