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

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Message-ID: <20150422110854.74988416@bahia.local>
Date:	Wed, 22 Apr 2015 11:08:54 +0200
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 7/8] vhost: feature to set the vring endianness

On Tue, 21 Apr 2015 20:25:03 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:
[ ... ]
> > > > @@ -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
> > > 
> > 
> > Since s.num is unsigned and I assume this won't change, what about
> > s.num > 1 as suggested by Cornelia ?
> 
> I just tried and gcc optimizes
> s.num != 0 && s.num != 1 to s.num > 1
> 
> The former will be more readable once we
> replace 0 and 1 with defines.
> 
> So ignore my advice, keep code as is but use defines.
> 

Ok.

[ ... ] 
> > > > --- 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?
> > > 
> > 
> > Ok. I'll put the defines here so that all the cross-endian stuff
> > lies in the same hunk. Is it ok for you ?
> 
> Fine.
> 
> > > > + * other values return EINVAL.
> 
> Pls also add a note saying that not all kernel configurations support this ioctl,
> but all configurations that support SET also support GET.
> 

Ok.

> > > > + */
> > > > +#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. */
> > > >  
> > > 
> 
> I'm inclined to think VHOST_SET_VRING_ENDIAN is a slightly better name.
> What do you think?
> 

Or VHOST_SET_VRING_CROSS_ENDIAN ? I like the idea to keep a hint that this
API is for cross-endian only... like the rest of this series.

--
Greg

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