Re: [PATCH v3 7/7] vhost: feature to set the vring endianness

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Tue, 7 Apr 2015 18:11:29 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Apr 07, 2015 at 02:19:31PM +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 ioctls introduced by this patch are for legacy only: virtio 1.0
> > devices are returned EPERM.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> >  drivers/vhost/Kconfig      |   10 ++++++++
> >  drivers/vhost/vhost.c      |   55 ++++++++++++++++++++++++++++++++++++++++++++
> >  drivers/vhost/vhost.h      |   17 +++++++++++++-
> >  include/uapi/linux/vhost.h |    5 ++++
> >  4 files changed, 86 insertions(+), 1 deletion(-)
> > 
> > Changes since v2:
> > - fixed typos in Kconfig description
> > - renamed vq->legacy_big_endian to vq->legacy_is_little_endian
> > - vq->legacy_is_little_endian reset to default in vhost_vq_reset()
> > - dropped VHOST_F_SET_ENDIAN_LEGACY feature
> > - dropped struct vhost_vring_endian from the user API (re-use
> >   struct vhost_vring_state instead)
> > - added VHOST_GET_VRING_ENDIAN_LEGACY ioctl
> > - introduced more helpers and stubs to avoid polluting the code with ifdefs
> > 
> > 
> > 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
> > +	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".
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 2ee2826..3529a3c 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_is_little_endian = virtio_legacy_is_little_endian();
> >  }
> >  
> >  static int vhost_worker(void *data)
> > @@ -630,6 +631,54 @@ 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_endian_legacy(struct vhost_virtqueue *vq,
> > +					  void __user *argp)
> > +{
> > +	struct vhost_vring_state s;
> > +
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		return -EPERM;
> > +
> > +	if (copy_from_user(&s, argp, sizeof(s)))
> > +		return -EFAULT;
> > +
> > +	vq->legacy_is_little_endian = !!s.num;
> > +	return 0;
> > +}
> > +
> > +static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  u32 idx,
> > +					  void __user *argp)
> > +{
> > +	struct vhost_vring_state s = {
> > +		.index = idx,
> > +		.num = vq->legacy_is_little_endian
> > +	};
> > +
> > +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > +		return -EPERM;
> > +
> > +	if (copy_to_user(argp, &s, sizeof(s)))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}
> > +#else
> > +static long vhost_set_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  void __user *argp)
> > +{
> > +	return 0;
> > +}
> > +
> > +static long vhost_get_vring_endian_legacy(struct vhost_virtqueue *vq,
> > +					  u32 idx,
> > +					  void __user *argp)
> > +{
> > +	return 0;
> > +}
> > +#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 +855,12 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> >  		} else
> >  			filep = eventfp;
> >  		break;
> > +	case VHOST_SET_VRING_ENDIAN_LEGACY:
> > +		r = vhost_set_vring_endian_legacy(vq, argp);
> > +		break;
> > +	case VHOST_GET_VRING_ENDIAN_LEGACY:
> > +		r = vhost_get_vring_endian_legacy(vq, idx, argp);
> > +		break;
> >  	default:
> >  		r = -ENOIOCTLCMD;
> >  	}
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 4e9a186..981ba06 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_is_little_endian;
> >  };
> >  
> >  struct vhost_dev {
> > @@ -173,11 +176,23 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
> >  	return vq->acked_features & (1ULL << bit);
> >  }
> >  
> > +#ifdef CONFIG_VHOST_SET_ENDIAN_LEGACY
> > +static inline bool vhost_legacy_is_little_endian(struct vhost_virtqueue *vq)
> > +{
> > +	return vq->legacy_is_little_endian;
> > +}
> > +#else
> > +static inline bool vhost_legacy_is_little_endian(struct vhost_virtqueue *vq)
> > +{
> > +	return virtio_legacy_is_little_endian();
> > +}
> > +#endif
> > +
> >  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 vhost_legacy_is_little_endian(vq);
> >  }
> >  
> >  /* Memory accessors */
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index bb6a5b4..1b01a72 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -103,6 +103,11 @@ 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) */
> > +/* num is 0 for big endian, other values mean little endian */
> > +#define VHOST_SET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_state)
> > +#define VHOST_GET_VRING_ENDIAN_LEGACY _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
> > +
> >  /* The following ioctls use eventfd file descriptors to signal and poll
> >   * for events. */
> >  
> 
> So if the config feature is enabled, you actually check two
> things: 1. feature 2. ioctl
> 
> Why not override is_le when we set VIRTIO_F_VERSION_1?
> I guess you are worried that setting a value and not being
> able to read it back is ugly. We can add a flag to track
> it though. So here's an idea
> 	I would probably rename to G/SET_BIG_ENDIAN then it's obvious
> 	it's legacy.  And just document that it's ignored with
> 	VIRTIO_F_VERSION_1.
> 
> 	make sure it's not changed when ring is running
> 
> 	simply set vq->is_le correctly when we start/stop the device
> 		vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
> 			!vq->user_be;
> 
> 	 static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> 	 {
> 		return vq->is_le;
> 	 }
> 
> 

I like the idea. I'll give it a try.

Thanks for your help Michael.

--
Greg