linux-kernel - Re: [PATCH v7 30/46] vhost/net: force len for TX to host endian

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Date:	Mon, 1 Dec 2014 14:33:34 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	rusty@....ibm.com, nab@...ux-iscsi.org, pbonzini@...hat.com,
	thuth@...ux.vnet.ibm.com, dahi@...ux.vnet.ibm.com,
	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH v7 30/46] vhost/net: force len for TX to host endian

On Mon, Dec 01, 2014 at 01:20:51PM +0100, Cornelia Huck wrote:
> On Sun, 30 Nov 2014 17:11:44 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > vhost/net keeps a copy of some used ring but (ab)uses length
> > field for internal house-keeping. This works because
> > for tx used length is always 0.
> > Suppress sparse errors: we use native endian-ness internally but never
> > expose it to guest.
> 
> I admit that I find this patch description hard to read :)
> 
> 
> "vhost/net keeps a copy of the used ring in host memory but (ab)uses
> the length field for internal house-keeping. This works because the
> length in the used ring for tx is always 0. In order to suppress sparse
> errors, we need to force native endianness."
> 
> ?

Yes. Add to this "These values are never exposed to guest."

> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > Reviewed-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  drivers/vhost/net.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 8dae2f7..dce5c58 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -48,15 +48,15 @@ MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
> >   * status internally; used for zerocopy tx only.
> >   */
> >  /* Lower device DMA failed */
> > -#define VHOST_DMA_FAILED_LEN	3
> > +#define VHOST_DMA_FAILED_LEN	((__force __virtio32)3)
> >  /* Lower device DMA done */
> > -#define VHOST_DMA_DONE_LEN	2
> > +#define VHOST_DMA_DONE_LEN	((__force __virtio32)2)
> >  /* Lower device DMA in progress */
> > -#define VHOST_DMA_IN_PROGRESS	1
> > +#define VHOST_DMA_IN_PROGRESS	((__force __virtio32)1)
> >  /* Buffer unused */
> > -#define VHOST_DMA_CLEAR_LEN	0
> > +#define VHOST_DMA_CLEAR_LEN	((__force __virtio32)0)
> > 
> > -#define VHOST_DMA_IS_DONE(len) ((len) >= VHOST_DMA_DONE_LEN)
> > +#define VHOST_DMA_IS_DONE(len) ((__force u32)(len) >= (__force u32)VHOST_DMA_DONE_LEN)
> > 
> >  enum {
> >  	VHOST_NET_FEATURES = VHOST_FEATURES |
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