linux-kernel - Re: [PATCH v4 26/42] vhost/net: force len for TX to host endian

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Date:	Wed, 26 Nov 2014 15:31:02 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	rusty@....ibm.com, nab@...ux-iscsi.org, pbonzini@...hat.com,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH v4 26/42] vhost/net: force len for TX to host endian

On Tue, 25 Nov 2014 18:43:14 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> We use native endian-ness internally but never
> expose it to guest.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...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)

I find these constants a bit confusing: What does __virtio32 mean
without the context of a vq or device?

> 
> -#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)

And here you cast it to a plain u32 again.

I looked at the final code, and you seem either to use the above
constants for .len or do a cpu_to_vhost32(). Wouldn't you need to
convert the constants as well?

> 
>  enum {
>  	VHOST_NET_FEATURES = VHOST_FEATURES |

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