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https://github.com/AuxXxilium/linux_dsm_epyc7002.git
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66aa0678ef
Current largesend and checksum offload feature in ibmveth driver,
- Source VM sends the TCP packets with ip_summed field set as
CHECKSUM_PARTIAL and TCP pseudo header checksum is placed in
checksum field
- CHECKSUM_PARTIAL flag in SKB will enable ibmveth driver to mark
"no checksum" and "checksum good" bits in transmit buffer descriptor
before the packet is delivered to pseries PowerVM Hypervisor
- If ibmveth has largesend capability enabled, transmit buffer descriptors
are market accordingly before packet is delivered to Hypervisor
(along with mss value for packets with length > MSS)
- Destination VM's ibmveth driver receives the packet with "checksum good"
bit set and so, SKB's ip_summed field is set with CHECKSUM_UNNECESSARY
- If "largesend" bit was on, mss value is copied from receive descriptor
into SKB's gso_size and other flags are appropriately set for
packets > MSS size
- The packet is now successfully delivered up the stack in destination VM
The offloads described above works fine for TCP communication among VMs in
the same pseries server ( VM A <=> PowerVM Hypervisor <=> VM B )
We are now enabling support for OVS in pseries PowerVM environment. One of
our requirements is to have ibmveth driver configured in "Trunk" mode, when
they are used with OVS. This is because, PowerVM Hypervisor will no more
bridge the packets between VMs, instead the packets are delivered to
IO Server which hosts OVS to bridge them between VMs or to external
networks (flow shown below),
VM A <=> PowerVM Hypervisor <=> IO Server(OVS) <=> PowerVM Hypervisor
<=> VM B
In "IO server" the packet is received by inbound Trunk ibmveth and then
delivered to OVS, which is then bridged to outbound Trunk ibmveth (shown
below),
Inbound Trunk ibmveth <=> OVS <=> Outbound Trunk ibmveth
In this model, we hit the following issues which impacted the VM
communication performance,
- Issue 1: ibmveth doesn't support largesend and checksum offload features
when configured as "Trunk". Driver has explicit checks to prevent
enabling these offloads.
- Issue 2: SYN packet drops seen at destination VM. When the packet
originates, it has CHECKSUM_PARTIAL flag set and as it gets delivered to
IO server's inbound Trunk ibmveth, on validating "checksum good" bits
in ibmveth receive routine, SKB's ip_summed field is set with
CHECKSUM_UNNECESSARY flag. This packet is then bridged by OVS (or Linux
Bridge) and delivered to outbound Trunk ibmveth. At this point the
outbound ibmveth transmit routine will not set "no checksum" and
"checksum good" bits in transmit buffer descriptor, as it does so only
when the ip_summed field is CHECKSUM_PARTIAL. When this packet gets
delivered to destination VM, TCP layer receives the packet with checksum
value of 0 and with no checksum related flags in ip_summed field. This
leads to packet drops. So, TCP connections never goes through fine.
- Issue 3: First packet of a TCP connection will be dropped, if there is
no OVS flow cached in datapath. OVS while trying to identify the flow,
computes the checksum. The computed checksum will be invalid at the
receiving end, as ibmveth transmit routine zeroes out the pseudo
checksum value in the packet. This leads to packet drop.
- Issue 4: ibmveth driver doesn't have support for SKB's with frag_list.
When Physical NIC has GRO enabled and when OVS bridges these packets,
OVS vport send code will end up calling dev_queue_xmit, which in turn
calls validate_xmit_skb.
In validate_xmit_skb routine, the larger packets will get segmented into
MSS sized segments, if SKB has a frag_list and if the driver to which
they are delivered to doesn't support NETIF_F_FRAGLIST feature.
This patch addresses the above four issues, thereby enabling end to end
largesend and checksum offload support for better performance.
- Fix for Issue 1 : Remove checks which prevent enabling TCP largesend and
checksum offloads.
- Fix for Issue 2 : When ibmveth receives a packet with "checksum good"
bit set and if its configured in Trunk mode, set appropriate SKB fields
using skb_partial_csum_set (ip_summed field is set with
CHECKSUM_PARTIAL)
- Fix for Issue 3: Recompute the pseudo header checksum before sending the
SKB up the stack.
- Fix for Issue 4: Linearize the SKBs with frag_list. Though we end up
allocating buffers and copying data, this fix gives
upto 4X throughput increase.
Note: All these fixes need to be dropped together as fixing just one of
them will lead to other issues immediately (especially for Issues 1,2 & 3).
Signed-off-by: Sivakumar Krishnasamy <ksiva@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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