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Re: [PATCH v9 13/20] virtio-net: Return an error when vhost cannot enabl


From: Akihiko Odaki
Subject: Re: [PATCH v9 13/20] virtio-net: Return an error when vhost cannot enable RSS
Date: Mon, 8 Apr 2024 10:29:49 +0900
User-agent: Mozilla Thunderbird

On 2024/04/08 6:46, Yuri Benditovich wrote:
On Wed, Apr 3, 2024 at 2:11 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:

vhost requires eBPF for RSS. When eBPF is not available, virtio-net
implicitly disables RSS even if the user explicitly requests it. Return
an error instead of implicitly disabling RSS if RSS is requested but not
available.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
---
  hw/net/virtio-net.c | 97 ++++++++++++++++++++++++++---------------------------
  1 file changed, 48 insertions(+), 49 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 61b49e335dea..3d53eba88cfc 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -793,9 +793,6 @@ static uint64_t virtio_net_get_features(VirtIODevice *vdev, 
uint64_t features,
          return features;
      }

-    if (!ebpf_rss_is_loaded(&n->ebpf_rss)) {
-        virtio_clear_feature(&features, VIRTIO_NET_F_RSS);
-    }
      features = vhost_net_get_features(get_vhost_net(nc->peer), features);
      vdev->backend_features = features;

@@ -3591,6 +3588,50 @@ static bool failover_hide_primary_device(DeviceListener 
*listener,
      return qatomic_read(&n->failover_primary_hidden);
  }

+static void virtio_net_device_unrealize(DeviceState *dev)
+{
+    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
+    VirtIONet *n = VIRTIO_NET(dev);
+    int i, max_queue_pairs;
+
+    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
+        virtio_net_unload_ebpf(n);
+    }
+
+    /* This will stop vhost backend if appropriate. */
+    virtio_net_set_status(vdev, 0);
+
+    g_free(n->netclient_name);
+    n->netclient_name = NULL;
+    g_free(n->netclient_type);
+    n->netclient_type = NULL;
+
+    g_free(n->mac_table.macs);
+    g_free(n->vlans);
+
+    if (n->failover) {
+        qobject_unref(n->primary_opts);
+        device_listener_unregister(&n->primary_listener);
+        migration_remove_notifier(&n->migration_state);
+    } else {
+        assert(n->primary_opts == NULL);
+    }
+
+    max_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
+    for (i = 0; i < max_queue_pairs; i++) {
+        virtio_net_del_queue(n, i);
+    }
+    /* delete also control vq */
+    virtio_del_queue(vdev, max_queue_pairs * 2);
+    qemu_announce_timer_del(&n->announce_timer, false);
+    g_free(n->vqs);
+    qemu_del_nic(n->nic);
+    virtio_net_rsc_cleanup(n);
+    g_free(n->rss_data.indirections_table);
+    net_rx_pkt_uninit(n->rx_pkt);
+    virtio_cleanup(vdev);
+}
+
  static void virtio_net_device_realize(DeviceState *dev, Error **errp)
  {
      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
@@ -3760,53 +3801,11 @@ static void virtio_net_device_realize(DeviceState *dev, 
Error **errp)

      net_rx_pkt_init(&n->rx_pkt);

-    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
-        virtio_net_load_ebpf(n);
-    }
-}
-
-static void virtio_net_device_unrealize(DeviceState *dev)
-{
-    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
-    VirtIONet *n = VIRTIO_NET(dev);
-    int i, max_queue_pairs;
-
-    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
-        virtio_net_unload_ebpf(n);
+    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS) &&

I disagree with this change of qemu behavior.
 From my point of view:
- this is not a major problem and it should not be a reason to stop VM execution
- it is enough to disable the RSS feature and continue working. Depending on
   other qemu parameters (number of queues, number of cpus) this might be just
   suboptimal. might be a minor problem and might be not a problem at all

The reasoning is that we shouldn't disable what the user explicitly requested. c.f., https://lore.kernel.org/all/20231102091717-mutt-send-email-mst@kernel.org/

- this change defines rss as _only_ feature whose absence breaks the VM start,
   _all_ other features are dropped silently and only rss is not. Why??

I'm following what QEMU does in the other places rather than what it does just in virtio-net. I have pointed out virtio-gpu raises errors in such a situation. c.f., https://lore.kernel.org/all/8880b6f9-f556-46f7-a191-eeec0fe208b0@daynix.com

- the series has a title 'Fixes and improvements' . This is not a fix and not an
   improvement, this is significant behavioral change that should be discussed 
in
   light of future plans regarding rss
- I suggest to remove this change from the series, submit it separately
   and discuss from all the sides

We should have already discussed about these matters; I responded all past replies in the previous versions months ago and had no update after that. Let's focus on matters that were not previously pointed out.

Regards,
Akihiko Odaki



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