On Wed, Feb 19, 2025 at 12:52:48PM +0200, Cosmin Ratiu wrote:
The cited commit fixed a software GSO bug with VXLAN + IPSec in tunnel mode. Unfortunately, it is slightly broader than necessary, as it also severely affects performance for Geneve + IPSec transport mode over a device capable of both HW GSO and IPSec crypto offload. In this case, xfrm_output unnecessarily triggers software GSO instead of letting the HW do it. In simple iperf3 tests over Geneve + IPSec transport mode over a back-2-back pair of NICs with MTU 1500, the performance was observed to be up to 6x worse when doing software GSO compared to leaving it to the hardware.
This commit makes xfrm_output only trigger software GSO in crypto offload cases for already encapsulated packets in tunnel mode, as not doing so would then cause the inner tunnel skb->inner_networking_header to be overwritten and break software GSO for that packet later if the device turns out to not be capable of HW GSO.
Taking a closer look at the conditions for the original bug, to better understand the reasons for this change:
- vxlan_build_skb -> iptunnel_handle_offloads sets inner_protocol and inner network header.
- then, udp_tunnel_xmit_skb -> ip_tunnel_xmit adds outer transport and network headers.
- later in the xmit path, xfrm_output -> xfrm_outer_mode_output -> xfrm4_prepare_output -> xfrm4_tunnel_encap_add overwrites the inner network header with the one set in ip_tunnel_xmit before adding the second outer header.
- __dev_queue_xmit -> validate_xmit_skb checks whether GSO segmentation needs to happen based on dev features. In the original bug, the hw couldn't segment the packets, so skb_gso_segment was invoked.
- deep in the .gso_segment callback machinery, __skb_udp_tunnel_segment tries to use the wrong inner network header, expecting the one set in iptunnel_handle_offloads but getting the one set by xfrm instead.
- a bit later, ipv6_gso_segment accesses the wrong memory based on that wrong inner network header.
With the new change, the original bug (or similar ones) cannot happen again, as xfrm will now trigger software GSO before applying a tunnel. This concern doesn't exist in packet offload mode, when the HW adds encapsulation headers. For the non-offloaded packets (crypto in SW), software GSO is still done unconditionally in the else branch.
Reviewed-by: Dragos Tatulea dtatulea@nvidia.com Reviewed-by: Yael Chemla ychemla@nvidia.com Fixes: a204aef9fd77 ("xfrm: call xfrm_output_gso when inner_protocol is set in xfrm_output") Signed-off-by: Cosmin Ratiu cratiu@nvidia.com
Applied, thanks Cosmin!