RE: [PATCH v6] hv_netvsc: add support for vlans in AF_PACKET mode

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: Sriram Krishnan <srirakr2@cisco.com>
> Sent: Wednesday, July 22, 2020 11:39 AM
> To: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> Wei Liu <wei.liu@kernel.org>
> Cc: mbumgard@cisco.com; ugm@cisco.com; nimm@cisco.com; xe-linux-
> external@cisco.com; David S. Miller <davem@davemloft.net>; Jakub Kicinski
> <kuba@kernel.org>; linux-hyperv@vger.kernel.org; netdev@vger.kernel.org;
> linux-kernel@vger.kernel.org
> Subject: [PATCH v6] hv_netvsc: add support for vlans in AF_PACKET mode
> 
> Vlan tagged packets are getting dropped when used with DPDK that uses
> the AF_PACKET interface on a hyperV guest.
> 
> The packet layer uses the tpacket interface to communicate the vlans
> information to the upper layers. On Rx path, these drivers can read the
> vlan info from the tpacket header but on the Tx path, this information
> is still within the packet frame and requires the paravirtual drivers to
> push this back into the NDIS header which is then used by the host OS to
> form the packet.
> 
> This transition from the packet frame to NDIS header is currently missing
> hence causing the host OS to drop the all vlan tagged packets sent by
> the drivers that use AF_PACKET (ETH_P_ALL) such as DPDK.
> 
> Here is an overview of the changes in the vlan header in the packet path:
> 
> The RX path (userspace handles everything):
>   1. RX VLAN packet is stripped by HOST OS and placed in NDIS header
>   2. Guest Kernel RX hv_netvsc packets and moves VLAN info from NDIS
>      header into kernel SKB
>   3. Kernel shares packets with user space application with PACKET_MMAP.
>      The SKB VLAN info is copied to tpacket layer and indication set
>      TP_STATUS_VLAN_VALID.
>   4. The user space application will re-insert the VLAN info into the frame
> 
> The TX path:
>   1. The user space application has the VLAN info in the frame.
>   2. Guest kernel gets packets from the application with PACKET_MMAP.
>   3. The kernel later sends the frame to the hv_netvsc driver. The only way
>      to send VLANs is when the SKB is setup & the VLAN is stripped from the
>      frame.
>   4. TX VLAN is re-inserted by HOST OS based on the NDIS header. If it sees
>      a VLAN in the frame the packet is dropped.
> 
> Cc: xe-linux-external@cisco.com
> Cc: Sriram Krishnan <srirakr2@cisco.com>
> Signed-off-by: Sriram Krishnan <srirakr2@cisco.com>

Thanks you!
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>