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    SubjectRe: [PATCH 2/3] vhost: add support for legacy virtio
    On Fri, Feb 20, 2015 at 11:14:47AM +0100, Greg Kurz wrote:
    > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
    > ---
    > drivers/vhost/vhost.h | 20 ++++++++++++++------
    > 1 file changed, 14 insertions(+), 6 deletions(-)
    >
    > Michael,
    >
    > The vhost_is_little_endian() helper adds unconditionnal overhead to fixed
    > endian architectures: that is all architectures except arm and ppc64. This
    > was addressed in QEMU with a TARGET_IS_BIENDIAN macro. Please give an
    > advice about how to address this in the vhost code.
    >
    > Thanks.

    I suggest creating a config option for this, default to off.
    When disabled the flag won't be set so userspace
    can discover it's availability.


    > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
    > index ce2c68e..21e7d6a 100644
    > --- a/drivers/vhost/vhost.h
    > +++ b/drivers/vhost/vhost.h
    > @@ -176,34 +176,42 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
    > return vq->acked_features & (1ULL << bit);
    > }
    >
    > +static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
    > +{
    > + if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
    > + return true;
    > + else
    > + return !vq->legacy_big_endian;
    > +}
    > +
    > /* Memory accessors */
    > static inline u16 vhost16_to_cpu(struct vhost_virtqueue *vq, __virtio16 val)
    > {
    > - return __virtio16_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
    > + return __virtio16_to_cpu(vhost_is_little_endian(vq), val);
    > }
    >
    > static inline __virtio16 cpu_to_vhost16(struct vhost_virtqueue *vq, u16 val)
    > {
    > - return __cpu_to_virtio16(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
    > + return __cpu_to_virtio16(vhost_is_little_endian(vq), val);
    > }
    >
    > static inline u32 vhost32_to_cpu(struct vhost_virtqueue *vq, __virtio32 val)
    > {
    > - return __virtio32_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
    > + return __virtio32_to_cpu(vhost_is_little_endian(vq), val);
    > }
    >
    > static inline __virtio32 cpu_to_vhost32(struct vhost_virtqueue *vq, u32 val)
    > {
    > - return __cpu_to_virtio32(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
    > + return __cpu_to_virtio32(vhost_is_little_endian(vq), val);
    > }
    >
    > static inline u64 vhost64_to_cpu(struct vhost_virtqueue *vq, __virtio64 val)
    > {
    > - return __virtio64_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
    > + return __virtio64_to_cpu(vhost_is_little_endian(vq), val);
    > }
    >
    > static inline __virtio64 cpu_to_vhost64(struct vhost_virtqueue *vq, u64 val)
    > {
    > - return __cpu_to_virtio64(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
    > + return __cpu_to_virtio64(vhost_is_little_endian(vq), val);
    > }
    > #endif


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