linux-kernel - Re: [PATCH v5 6/8] virtio: add explicit big-endian support to memory accessors

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Date:	Fri, 24 Apr 2015 09:56:26 +0200
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	Thomas Huth <thuth@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 6/8] virtio: add explicit big-endian support to
 memory accessors

On Fri, 24 Apr 2015 09:04:21 +0200
Cornelia Huck <cornelia.huck@...ibm.com> wrote:
> On Thu, 23 Apr 2015 21:27:19 +0200
> Thomas Huth <thuth@...hat.com> wrote:
> 

Thomas's e-mail did not make it to my mailbox... weird. :-\

> > On Thu, 23 Apr 2015 17:29:06 +0200
> > Greg Kurz <gkurz@...ux.vnet.ibm.com> wrote:
> > 
> > > The current memory accessors logic is:
> > > - little endian if little_endian
> > > - native endian (i.e. no byteswap) if !little_endian
> > > 
> > > If we want to fully support cross-endian vhost, we also need to be
> > > able to convert to big endian.
> > > 
> > > Instead of changing the little_endian argument to some 3-value enum, this
> > > patch changes the logic to:
> > > - little endian if little_endian
> > > - big endian if !little_endian
> > > 
> > > The native endian case is handled by all users with a trivial helper. This
> > > patch doesn't change any functionality, nor it does add overhead.
> > > 
> > > Signed-off-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>
> > > ---
> > > 
> > > Changes since v4:
> > > - style fixes (I have chosen if ... else in most places to stay below
> > >   80 columns, with the notable exception of the vhost helper which gets
> > >   shorten in a later patch)
> > > 
> > >  drivers/net/macvtap.c            |    5 ++++-
> > >  drivers/net/tun.c                |    5 ++++-
> > >  drivers/vhost/vhost.h            |    2 +-
> > >  include/linux/virtio_byteorder.h |   24 ++++++++++++++----------
> > >  include/linux/virtio_config.h    |    5 ++++-
> > >  include/linux/vringh.h           |    2 +-
> > >  6 files changed, 28 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> > > index a2f2958..6cf6b3e 100644
> > > --- a/drivers/net/macvtap.c
> > > +++ b/drivers/net/macvtap.c
> > > @@ -51,7 +51,10 @@ struct macvtap_queue {
> > >  
> > >  static inline bool macvtap_is_little_endian(struct macvtap_queue *q)
> > >  {
> > > -	return q->flags & MACVTAP_VNET_LE;
> > > +	if (q->flags & MACVTAP_VNET_LE)
> > > +		return true;
> > > +	else
> > > +		return virtio_legacy_is_little_endian();
> > 
> > simply:
> > 
> > 	return (q->flags & MACVTAP_VNET_LE) ||
> > 	       virtio_legacy_is_little_endian();
> > 
> > ?
> 
> ISTR that MST preferred the current notation, but I like either your
> way or ?: (with linebreak) even better.
> 

MST did not like the initial notation I had used actually. FWIW I
like the simplicity of Thomas's suggestion... even better than ?:
which is:
         some_LE_check() ? true : some_default_endianness()

> > 
> > >  }
> > >  
> (...)
> > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > index 6a49960..954c657 100644
> > > --- a/drivers/vhost/vhost.h
> > > +++ b/drivers/vhost/vhost.h
> > > @@ -175,7 +175,7 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
> > >  
> > >  static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> > >  {
> > > -	return vhost_has_feature(vq, VIRTIO_F_VERSION_1);
> > > +	return vhost_has_feature(vq, VIRTIO_F_VERSION_1) ? true : virtio_legacy_is_little_endian();
> > >  }
> > 
> > That line is way longer than 80 characters ... may I suggest to switch
> > at least here to:
> > 
> > 	return vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
> > 	       virtio_legacy_is_little_endian();
> 
> I think the line will collapse in a further patch anyway.
> 

Yes, as mentionned in the changelog :)

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