vdpa: Avoid forwarding large CVQ command failures

vdpa: Avoid forwarding large CVQ command failures

Due to the size limitation of the out buffer sent to the vdpa device,
which is determined by vhost_vdpa_net_cvq_cmd_len(), excessive CVQ
command is truncated in QEMU. As a result, the vdpa device rejects
this flawd CVQ command.

However, the problem is that, the VIRTIO_NET_CTRL_MAC_TABLE_SET
CVQ command has a variable length, which may exceed
vhost_vdpa_net_cvq_cmd_len() if the guest sets more than
`MAC_TABLE_ENTRIES` MAC addresses for the filter table.

This patch solves this problem by following steps:

  * Increase the out buffer size to vhost_vdpa_net_cvq_cmd_page_len(),
which represents the size of the buffer that is allocated and mmaped.
This ensures that everything works correctly as long as the guest
sets fewer than `(vhost_vdpa_net_cvq_cmd_page_len() -
sizeof(struct virtio_net_ctrl_hdr)
- 2 * sizeof(struct virtio_net_ctrl_mac)) / ETH_ALEN` MAC addresses.
    Considering the highly unlikely scenario for the guest setting
more than that number of MAC addresses for the filter table, this
should work fine for the majority of cases.

  * If the CVQ command exceeds vhost_vdpa_net_cvq_cmd_page_len(),
instead of directly sending this CVQ command, QEMU should send
a VIRTIO_NET_CTRL_RX_PROMISC CVQ command to vdpa device. Addtionally,
a fake VIRTIO_NET_CTRL_MAC_TABLE_SET command including
(`MAC_TABLE_ENTRIES` + 1) non-multicast MAC addresses and
(`MAC_TABLE_ENTRIES` + 1) multicast MAC addresses should be provided
to the device model.
    By doing so, the vdpa device turns promiscuous mode on, aligning
with the VirtIO standard. The device model marks
`n->mac_table.uni_overflow` and `n->mac_table.multi_overflow`,
which aligns with the state of the vdpa device.

Note that the bug cannot be triggered at the moment, since
VIRTIO_NET_F_CTRL_RX feature is not enabled for SVQ.

Fixes: 7a7f87e94c ("vdpa: Move command buffers map to start of net device")
Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <267e15e4eed2d7aeb9887f193da99a13d22a2f1d.1688743107.git.yin31149@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>