On Mon, Oct 30, 2023 at 7:14 AM Akihiko Odaki <akihiko.od...@daynix.com>
wrote:
> virtio-net implements software RSS but does not enable it. Enable it
> when RSS is requested, but the eBPF implementation is not available.
> We also check if vhost is in use in such a case since software RSS is
> incompatible with vhost. A warning will be emitted when falling back to
> software RSS since it provides no performance benefit.
>
>
Can you please elaborate what is wrong from your point of view in the
existing implementation?
In general it does (IMO) what you describe.
I'd like to note several things:
- The "vhost=off" is in fact the fallback mode (libvirt default is vhost=on)
- The main goal of software RSS was to provide a reference for RSS
implementation for future virtio-net hardware
- Performance benefit of software RSS (as well as implementation in EBPF)
is delivery of each packet to proper virtqueue/CPU and avoiding packet
rescheduling in the guest
- The best thing (IMO) would be to implement hash delivery with vhost=on,
i.e. in EBPF and as soon as this is possible - rely on the hardware/host
capabilities and stop calculating the hash in the guest driver (as we do
today in Windows)
> Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com>
> ---
> hw/net/virtio-net.c | 21 +++++++++++----------
> 1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 7bb91617d0..1fa020d905 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1260,10 +1260,12 @@ static bool virtio_net_attach_epbf_rss(VirtIONet
> *n)
>
> if (!ebpf_rss_set_all(&n->ebpf_rss, &config,
> n->rss_data.indirections_table,
> n->rss_data.key)) {
> + warn_report("Failed to configure eBPF RSS");
> return false;
> }
>
> if (!virtio_net_attach_ebpf_to_backend(n->nic,
> n->ebpf_rss.program_fd)) {
> + warn_report("Failed to attach eBPF to backend");
> return false;
> }
>
> @@ -1278,16 +1280,10 @@ static void virtio_net_detach_epbf_rss(VirtIONet
> *n)
> static void virtio_net_commit_rss_config(VirtIONet *n)
> {
> if (n->rss_data.enabled) {
> - n->rss_data.enabled_software_rss = n->rss_data.populate_hash;
> + n->rss_data.enabled_software_rss = n->rss_data.populate_hash ||
> + !virtio_net_attach_epbf_rss(n);
> if (n->rss_data.populate_hash) {
> virtio_net_detach_epbf_rss(n);
> - } else if (!virtio_net_attach_epbf_rss(n)) {
> - if (get_vhost_net(qemu_get_queue(n->nic)->peer)) {
> - warn_report("Can't load eBPF RSS for vhost");
> - } else {
> - warn_report("Can't load eBPF RSS - fallback to software
> RSS");
> - n->rss_data.enabled_software_rss = true;
> - }
> }
>
> trace_virtio_net_rss_enable(n->rss_data.hash_types,
> @@ -3747,8 +3743,13 @@ static void virtio_net_device_realize(DeviceState
> *dev, Error **errp)
>
> if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS) &&
> !virtio_net_load_ebpf(n)) {
> - error_setg(errp, "Can't load eBPF RSS");
> - virtio_net_device_unrealize(dev);
> + if (get_vhost_net(nc->peer)) {
> + error_setg(errp, "Can't load eBPF RSS for vhost");
> + virtio_net_device_unrealize(dev);
> + return;
> + }
> +
> + warn_report_once("Can't load eBPF RSS - fallback to software
> RSS");
> }
> }
>
> --
> 2.42.0
>
>
