Linux userland API discussions

* Re: [PATCH v2 7/7] vhost: feature to set the vring endianness
From: Michael S. Tsirkin @ 2015-04-02 14:20 UTC (permalink / raw)
  To: Greg Kurz; +Cc: linux-api, linux-kernel, kvm, virtualization
In-Reply-To: <20150402131713.24676.9924.stgit@bahia.lab.toulouse-stg.fr.ibm.com>

On Thu, Apr 02, 2015 at 03:17:13PM +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).
> 
> If cross-endian support is compiled in, vhost abvertises a new feature
> to be negotiated with userspace. If userspace acknowledges the feature,
> it can inform vhost about the endianness to use with a new ioctl.
> 
> This feature is mutually exclusive with virtio 1.0. Even if the vhost device
> advertises virtio 1.0 and legacy cross-endian features, it cannot receive
> aknowledgement for both at the same time.
> 
> Hot paths are being preserved from any penalty when the config option is
> disabled or when virtio 1.0 is being used.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  drivers/vhost/Kconfig      |   10 ++++++++++
>  drivers/vhost/net.c        |    5 +++++
>  drivers/vhost/scsi.c       |    4 ++++
>  drivers/vhost/test.c       |    4 ++++
>  drivers/vhost/vhost.c      |   19 +++++++++++++++++++
>  drivers/vhost/vhost.h      |   13 ++++++++++++-
>  include/uapi/linux/vhost.h |   10 ++++++++++
>  7 files changed, 64 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 017a1e8..5bb8da9 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
> +	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).
> +
> +	  If unsure, say "n".

"N"

> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 2bbfc25..5274a44 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1011,6 +1011,11 @@ static int vhost_net_set_features(struct vhost_net *n, u64 features)
>  		vhost_hlen = 0;
>  		sock_hlen = hdr_len;
>  	}
> +
> +	if (features & ((1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> +			(1ULL << VIRTIO_F_VERSION_1)))
> +		return -EINVAL;
> +
>  	mutex_lock(&n->dev.mutex);
>  	if ((features & (1 << VHOST_F_LOG_ALL)) &&
>  	    !vhost_log_access_ok(&n->dev)) {
> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index 71df240..b53e9c2 100644
> --- a/drivers/vhost/scsi.c
> +++ b/drivers/vhost/scsi.c
> @@ -1544,6 +1544,10 @@ static int vhost_scsi_set_features(struct vhost_scsi *vs, u64 features)
>  	if (features & ~VHOST_SCSI_FEATURES)
>  		return -EOPNOTSUPP;
>  
> +	if (features & ((1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> +			(1ULL << VIRTIO_F_VERSION_1)))
> +		return -EINVAL;
> +
>  	mutex_lock(&vs->dev.mutex);
>  	if ((features & (1 << VHOST_F_LOG_ALL)) &&
>  	    !vhost_log_access_ok(&vs->dev)) {
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index d9c501e..cfefdad 100644
> --- a/drivers/vhost/test.c
> +++ b/drivers/vhost/test.c
> @@ -243,6 +243,10 @@ static int vhost_test_set_features(struct vhost_test *n, u64 features)
>  {
>  	struct vhost_virtqueue *vq;
>  
> +	if (features & ((1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> +			(1ULL << VIRTIO_F_VERSION_1)))
> +		return -EINVAL;
> +
>  	mutex_lock(&n->dev.mutex);
>  	if ((features & (1 << VHOST_F_LOG_ALL)) &&
>  	    !vhost_log_access_ok(&n->dev)) {

Above seems to prevent users from specifying either
VIRTIO_F_VERSION_1 or VHOST_F_SET_ENDIAN_LEGACY.
Does it actually work?

> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 2ee2826..60a0f32 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->legacy_big_endian = false;
>  }
>  
>  static int vhost_worker(void *data)
> @@ -806,6 +807,24 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  		} else
>  			filep = eventfp;
>  		break;
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +	case VHOST_SET_VRING_ENDIAN_LEGACY:
> +	{
> +		struct vhost_vring_endian e;
> +
> +		if (!vhost_has_feature(vq, VHOST_F_SET_ENDIAN_LEGACY)) {
> +			r = -EINVAL;
> +			break;
> +		}
> +
> +		if (copy_from_user(&e, argp, sizeof(e))) {
> +			r = -EFAULT;
> +			break;
> +		}
> +		vq->legacy_big_endian = e.is_big_endian;
> +		break;
> +	}
> +#endif
>  	default:
>  		r = -ENOIOCTLCMD;
>  	}
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 4e9a186..d50881d 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -106,6 +106,9 @@ struct vhost_virtqueue {
>  	/* Log write descriptors */
>  	void __user *log_base;
>  	struct vhost_log *log;
> +
> +	/* We need to know the device endianness with legacy virtio. */
> +	bool legacy_big_endian;
>  };
>  
>  struct vhost_dev {
> @@ -165,7 +168,11 @@ enum {
>  	VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
>  			 (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
>  			 (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> -			 (1ULL << VHOST_F_LOG_ALL),
> +			 (1ULL << VHOST_F_LOG_ALL) |
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +			 (1ULL << VHOST_F_SET_ENDIAN_LEGACY) |
> +#endif
> +			 0ULL,
>  };
>  
>  static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
> @@ -177,6 +184,10 @@ static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
>  {
>  	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
>  		return true;
> +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> +	if (vhost_has_feature(vq, VHOST_F_SET_ENDIAN_LEGACY))
> +		return !vq->legacy_big_endian;

why do we need to check the feature bit?
How about simple
	return !vq->legacy_big_endian;
here?
All you need to do is set legacy_big_endian to
!virtio_legacy_is_little_endian() on reset.
Maybe rename to legacy_is_little_endian so you don't
need to reverse the logic.

> +#endif
>  	return virtio_legacy_is_little_endian();
>  }
>  
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index bb6a5b4..09d2a48 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -47,6 +47,11 @@ struct vhost_vring_addr {
>  	__u64 log_guest_addr;
>  };
>  
> +struct vhost_vring_endian {
> +	unsigned int index;
> +	bool is_big_endian;

bool in uapi is a bad idea.
Generally, I think you can use vhost_vring_state here.

> +};
> +
>  struct vhost_memory_region {
>  	__u64 guest_phys_addr;
>  	__u64 memory_size; /* bytes */
> @@ -103,6 +108,9 @@ 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 endianness for the ring (legacy virtio only) */
> +#define VHOST_SET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_endian)
> +
>  /* The following ioctls use eventfd file descriptors to signal and poll
>   * for events. */
>  

You also need a GET ioctl, as a matter of policy.

> @@ -126,6 +134,8 @@ struct vhost_memory {
>  #define VHOST_F_LOG_ALL 26
>  /* vhost-net should add virtio_net_hdr for RX, and strip for TX packets. */
>  #define VHOST_NET_F_VIRTIO_NET_HDR 27
> +/* the vring endianness can be explicitly set (legacy virtio only). */
> +#define VHOST_F_SET_ENDIAN_LEGACY 28
>  
>  /* VHOST_SCSI specific definitions */

VHOST_F_SET_ENDIAN_LEGACY doesn't seem too useful.
Is this so userspace can detect kernel configuration?
I think probing VHOST_SET_VRING_ENDIAN_LEGACY should
be sufficient for this.

-- 
MST

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