Re: Supporting RSS with DPDK in a VM

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Matheus,

On 7/28/23 16:12, Thomas Monjalon wrote:
> Hello,
> 
> You can have packets distributed to multiple queues without RSS.
> 
> If you really wants to enable the RSS algorithms,
> it seems not supported for now with vhost_user.
> It can be enabled with vhost running in the kernel:
> https://qemu.readthedocs.io/en/latest/devel/ebpf_rss.html

Thomas is right, DPDK Virtio PMD supports RSS, but DPDK Vhost-user does
not (yet).

However, as you seem to be using a fairly recent OVS-DPDK version, you 
can enable Hash-based Tx packet steering mode on the vhostuser ports.

It has been introduced in OVS v2.17:
https://docs.openvswitch.org/en/latest/topics/userspace-tx-steering/

It is usefull if you have less PMD threads in OVS than the number of
queues in your Virtio device. Traffic will be distributed on the queues
based on the 5-tuple hash computed for OVS switching. The downside is
that this is not configurable as RSS though (Key not configurable, no
filters, ...).

Regards,
Maxime

> 
> 26/07/2023 00:58, Matheus Stolet:
>> Hello,
>>
>> I am trying to run a DPDK application with RSS enabled so that I can
>> have multiple rx queues. This application is running inside a VM. This
>> VM is hosted by QEMU using KVM acceleration and OvS with DPDK and
>> vhost-user are used in the backend. So to clarify things there are two
>> DPDK portions to this. The first is the DPDK portion used by OvS that
>> bypasses the host operating system. This is working fine. The other is a
>> DPDK application inside the virtual machine that will bypass the guest
>> operating system. This is where I am having trouble with.
>>
>> When I set rte_eth_conf.rxmode.mq_mode = ETH_MQ_RX_RSS in my application
>> I get the following errors:
>>     Warning: NIC does not support all requested RSS hash functions.
>>     virtio_dev_configure(): RSS support requested but not supported by the
>> device
>>     Port0 dev_configure = -95
>>
>> I setup my VM in QEMU to have mq=on and queues=10. I also set the number
>> of rx_queues when creating the vhost port using ovs to 10. Before
>> binding the interface to DPDK, I used ethtool to verify if the network
>> interface was actually setup to have multiple queues.
>>
>> Running the 'ethtool -l enps02' command yields the following output:
>>     Pre-set maximums:
>>     RX:             0
>>     TX:             0
>>     Other:          0
>>     Combined:       10
>>     Current hardware settings:
>>     RX:             0
>>     TX:             0
>>     Other:          0
>>     Combined:       10
>>
>>   From my understanding the combined values indicate that the interface
>> was properly setup to have multiple queues, so why am I getting the
>> unsupported RSS error? Are there other configuration steps that I have
>> to take to get this to work? Is RSS with DPDK not supported at all
>> inside a VM at the moment? Perhaps the "Port0 dev_configure() = -95"
>> error means something else? Without the receive side scaling turned on
>> my application is not able to achieve the desired throughput and won't
>> scale when I assign more cores to the application.
>>
>> Versions:
>> VM
>> DPDK: 21.11.4
>> Kernel: 5.4.0-148-generic
>> Distribution: Ubuntu 20.04
>>
>> Host
>> DPDK: 21.11.4
>> QEMU: 8.0.90
>> OvS: 3.0.5
>> Kernel: 5.15.111.1.amd64-smp
>> Distribution: Debian 11
>>
> 
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