Question about virtio-net: implement RX RSS processing

[Vincent Li]

Hi,

I noticed the [PULL V2 02/33] virtio-net: implement RX RSS processing 
https://marc.info/?l=qemu-devel&m=159248675403246&w=2 and cloned  
https://github.com/jasowang/qemu.git tags/net-pull-request for testing the 
RX RSS feature, but I am not clear how to test this feature and see if it 
meets my needs.

I am running F-Stack Nginx applications 
https://github.com/F-Stack/f-stack/tree/dev/app/nginx-1.16.1 in KVM guest, 
F-Stack is FreeBSD TCP/IP stack ported to support DPDK, and F-Stack set 
RSS mode as code below 
https://github.com/F-Stack/f-stack/blob/dev/lib/ff_dpdk_if.c#L605

     /* Set RSS mode */
     uint64_t default_rss_hf = ETH_RSS_PROTO_MASK;
     port_conf.rxmode.mq_mode = ETH_MQ_RX_RSS;
     port_conf.rx_adv_conf.rss_conf.rss_hf = default_rss_hf;
     if (dev_info.hash_key_size == 52) {
         port_conf.rx_adv_conf.rss_conf.rss_key = default_rsskey_52bytes;
         port_conf.rx_adv_conf.rss_conf.rss_key_len = 52;
         use_rsskey_52bytes = 1;
     } else {
         port_conf.rx_adv_conf.rss_conf.rss_key = default_rsskey_40bytes;
         port_conf.rx_adv_conf.rss_conf.rss_key_len = 40;
     }
     port_conf.rx_adv_conf.rss_conf.rss_hf &= dev_info.flow_type_rss_offloads;
     if (port_conf.rx_adv_conf.rss_conf.rss_hf != ETH_RSS_PROTO_MASK) {
         printf("Port %u modified RSS hash function based on hardware support,"
         "requested:%#"PRIx64" configured:%#"PRIx64"\n",
         port_id, default_rss_hf, port_conf.rx_adv_conf.rss_conf.rss_hf);
     }

But  DPDK virtio PMD does not support RSS as below commit shows:

commit 13b3137f3b7c8f866947a9b34e06a8aec0d084f7
Author: Dilshod Urazov 
Date:   Wed Oct 9 13:32:07 2019 +0100

    net/virtio: reject unsupported Rx multi-queue modes
    
    This driver supports none of DCB, RSS or VMDQ modes, therefore must
    check and return error if configured incorrectly.
    
    Virtio can distribute Rx packets across multi-queue, but there is
    no controls (algorithm, redirection table, hash function) except
    number of Rx queues and ETH_MQ_RX_NONE is the best fit meaning
    no method is enforced on how to route packets to MQs.
    
    Fixes: c1f86306a026 ("virtio: add new driver")
    Cc: stable@dpdk.org
    
    Signed-off-by: Dilshod Urazov 
    Signed-off-by: Andrew Rybchenko 
    Reviewed-by: Maxime Coquelin 

diff --git a/drivers/net/virtio/virtio_ethdev.c 
           b/drivers/net/virtio/virtio_ethdev.c
index 0a2ed2e50..76bd40a3e 100644
--- a/drivers/net/virtio/virtio_ethdev.c
+++ b/drivers/net/virtio/virtio_ethdev.c
@@ -2066,6 +2066,13 @@ virtio_dev_configure(struct rte_eth_dev *dev)
        PMD_INIT_LOG(DEBUG, "configure");
        req_features = VIRTIO_PMD_DEFAULT_GUEST_FEATURES;
 
+       if (rxmode->mq_mode != ETH_MQ_RX_NONE) {
+               PMD_DRV_LOG(ERR,
+                       "Unsupported Rx multi queue mode %d",
+                       rxmode->mq_mode);
+               return -EINVAL;
+       }
+
        if (dev->data->dev_conf.intr_conf.rxq) {
                ret = virtio_init_device(dev, hw->req_guest_features);
                if (ret < 0)



So the problem is I can't run F-Stack Applications in KVM/Qemu guest with 
multi queue/vCPU/RSS  support, this problem seems apply to DPDK TCP 
applications require multi queue/RSS support in KVM/Qemu guest, for 
example mTCP https://github.com/mtcp-stack/mtcp I tested has similar 
problem.

I am not clear on the picture of how everything work together for  this 
virtio-net RSS feature.

I have read following blogs

https://www.redhat.com/en/blog/introduction-virtio-networking-and-vhost-net
https://www.redhat.com/en/blog/how-vhost-user-came-being-virtio-networking-and-dpdk

Someone told me that in order for DPDK frond end virtio PMD in guest support 
RSS, the backend 
also needs to support RSS, including vhost-net and vhost-user, it should 
have nothing to do with this Qemu virtio-net RSS, is that correct?  if 
correct, I have following questions:

1, What is the use case for this Qemu virtio-net RSS?
2, How to test the use case?
3, Are there any plan to improve vhost-net/vhost-user, DPDK virtio PMD to 
support RSS?

For 3,  I think this is important for KVM/Qemu/OVS-DPDK/Vhost-net environment 
for DPDK TCP/UDP applications. 

Note I have no problem running F-Stack or mTCP applications in VMware ESXi 
guest environment with multi queue/vCPU/RSS requirement since DPDK vmxnet3 
PMD support RSS and I assume VMware ESXi backend support RSS, so it looks 
VMware has 
advantage over this.

Thank you for your patience to read this email 

Regards,

Vincent