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

From: Cornelia Huck
Date: Thu Jan 21 2016 - 04:55:59 EST

On Wed, 13 Jan 2016 18:09:41 +0100
Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> ---
> drivers/vhost/vhost.c | 28 +++++++++++++++++++---------
> 1 file changed, 19 insertions(+), 9 deletions(-)

Reviewed-by: Cornelia Huck <cornelia.huck@xxxxxxxxxx>