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

On Wed, 13 Jan 2016 18:09:41 +0100
Greg Kurz [off-list ref] 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 <redacted>
---
 drivers/vhost/vhost.c |   28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

Reviewed-by: Cornelia Huck <redacted>