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Re: [Qemu-devel] [PATCH] net: don't set native endianness
From: |
Greg Kurz |
Subject: |
Re: [Qemu-devel] [PATCH] net: don't set native endianness |
Date: |
Wed, 14 Oct 2015 15:54:32 +0200 |
On Wed, 14 Oct 2015 12:26:58 +0300
"Michael S. Tsirkin" <address@hidden> wrote:
> commit 5be7d9f1b1452613b95c6ba70b8d7ad3d0797991
> vhost-net: tell tap backend about the vnet endianness
> makes vhost net always try to set LE - even if that matches the
> native endian-ness.
>
> This makes it fail on older kernels on x86 without TUNSETVNETLE support.
>
Since qemu_set_vnet_le() is only called from vhost_net_set_vnet_endian():
if (virtio_vdev_has_feature(dev, VIRTIO_F_VERSION_1) ||
(virtio_legacy_is_cross_endian(dev) && !virtio_is_big_endian(dev))) {
r = qemu_set_vnet_le(peer, set);
if (r) {
error_report("backend does not support LE vnet headers");
}
and virtio_legacy_is_cross_endian() is { return false; } on x86, I guess this
is about virtio 1.0, right ?
> To fix, make qemu_set_vnet_le/qemu_set_vnet_be skip the
> ioctl if it matches the host endian-ness.
>
The qemu_set_vnet_le() change indeed makes sense since it fixes a bug.
I am not so sure about qemu_set_vnet_be() since it is only called in the
case we have a ppc64le host and a legacy ppc64 guest, in which case
HOST_WORDS_BIGENDIAN is not defined...
IMHO it is better to rework the logic so that we only call qemu_set_vnet_*
when it is needed. There are only 3 cases actually:
- BE host with virtio 1 needs vnet_le
- BE host with legacy LE needs vnet_le
- LE host with legacy BE needs vnet_be
Something like:
static inline bool vhost_net_needs_vnet_le(VirtIODevice *dev)
{
#if defined(HOST_WORDS_BIGENDIAN)
return
virtio_vdev_has_feature(dev, VIRTIO_F_VERSION_1) ||
!virtio_is_big_endian(dev);
#else
return false;
#endif
}
static inline bool vhost_net_needs_vnet_be(VirtIODevice *dev)
{
#if defined(HOST_WORDS_BIGENDIAN)
return false;
#else
return virtio_is_big_endian(dev);
#endif
}
static int vhost_net_set_vnet_endian(VirtIODevice *dev, NetClientState *peer,
bool set)
{
int r = 0;
if (vhost_net_needs_vnet_le(dev)) {
r = qemu_set_vnet_le(peer, set);
if (r) {
error_report("backend does not support LE vnet headers");
}
} else if (vhost_net_needs_vnet_be(dev)) {
r = qemu_set_vnet_be(peer, set);
if (r) {
error_report("backend does not support BE vnet headers");
}
}
return r;
}
Cheers.
--
Greg
> Reported-by: Marcel Apfelbaum <address@hidden>
> Cc: Greg Kurz <address@hidden>
> Cc: address@hidden
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
> net/net.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/net/net.c b/net/net.c
> index 28a5597..8e96011 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -517,20 +517,28 @@ void qemu_set_vnet_hdr_len(NetClientState *nc, int len)
>
> int qemu_set_vnet_le(NetClientState *nc, bool is_le)
> {
> +#ifdef HOST_WORDS_BIGENDIAN
> if (!nc || !nc->info->set_vnet_le) {
> return -ENOSYS;
> }
>
> return nc->info->set_vnet_le(nc, is_le);
> +#else
> + return 0;
> +#endif
> }
>
> int qemu_set_vnet_be(NetClientState *nc, bool is_be)
> {
> +#ifdef HOST_WORDS_BIGENDIAN
> + return 0;
> +#else
> if (!nc || !nc->info->set_vnet_be) {
> return -ENOSYS;
> }
>
> return nc->info->set_vnet_be(nc, is_be);
> +#endif
> }
>
> int qemu_can_send_packet(NetClientState *sender)