Re: [PATCH 1/2] vhost: helpers to enable/disable vring endianness

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed, 13 Jan 2016 18:09:41 +0100
Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> The default use case for vhost is when the host and the vring have the
> same endianness (default native endianness). But there are cases where
> they differ and vhost should byteswap when accessing the vring:
> - the host is big endian and the vring comes from a virtio 1.0 device
>   which is always little endian
> - the architecture is bi-endian and the vring comes from a legacy virtio
>   device with a different endianness than the endianness of the host (aka
>   legacy cross-endian)
> 
> These cases are handled by the vq->is_le and the optional vq->user_be,
> with the following logic:
> - if none of the fields is enabled, vhost access the vring without byteswap
> - if the vring is virtio 1.0 and the host is big endian, vq->is_le is
>   enabled to enforce little endian access to the vring
> - if the vring is legacy cross-endian, userspace enables vq->user_be
>   to inform vhost about the vring endianness. This endianness is then
>   enforced for vring accesses through vq->is_le again
> 
> The logic is unclear in the current code.
> 
> This patch introduces helpers with explicit enable and disable semantics,
> for better clarity.
> 
> No behaviour change.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  drivers/vhost/vhost.c |   28 +++++++++++++++++++---------
>  1 file changed, 19 insertions(+), 9 deletions(-)

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>