If the user/mgmt app passed in a set of pre-opened FDs for eBPF RSS,
then it is expecting QEMU to use them. Any failure to do so must be
considered a fatal error and propagated back up the stack, otherwise
deployment mistakes will not be detectable in a prompt manner. When
not using pre-opened FDs, then eBPF RSS is tried on a "best effort"
basis only and thus fallback to software RSS is valid.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 hw/net/virtio-net.c | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 558fc62844..f2690390c1 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1316,28 +1316,27 @@ static void virtio_net_disable_rss(VirtIONet *n)
     virtio_net_commit_rss_config(n);
 }
 
-static bool virtio_net_load_ebpf_fds(VirtIONet *n)
+static bool virtio_net_load_ebpf_fds(VirtIONet *n, Error **errp)
 {
     int fds[EBPF_RSS_MAX_FDS] = { [0 ... EBPF_RSS_MAX_FDS - 1] = -1};
     int ret = true;
     int i = 0;
 
     if (n->nr_ebpf_rss_fds != EBPF_RSS_MAX_FDS) {
-        warn_report("Expected %d file descriptors but got %d",
-                    EBPF_RSS_MAX_FDS, n->nr_ebpf_rss_fds);
-       return false;
-   }
+        error_setg(errp, "Expected %d file descriptors but got %d",
+                   EBPF_RSS_MAX_FDS, n->nr_ebpf_rss_fds);
+        return false;
+    }
 
     for (i = 0; i < n->nr_ebpf_rss_fds; i++) {
-        fds[i] = monitor_fd_param(monitor_cur(), n->ebpf_rss_fds[i],
-                                  &error_warn);
+        fds[i] = monitor_fd_param(monitor_cur(), n->ebpf_rss_fds[i], errp);
         if (fds[i] < 0) {
             ret = false;
             goto exit;
         }
     }
 
-    ret = ebpf_rss_load_fds(&n->ebpf_rss, fds[0], fds[1], fds[2], fds[3], NULL);
+    ret = ebpf_rss_load_fds(&n->ebpf_rss, fds[0], fds[1], fds[2], fds[3], errp);
 
 exit:
     if (!ret) {
@@ -1349,13 +1348,15 @@ exit:
     return ret;
 }
 
-static bool virtio_net_load_ebpf(VirtIONet *n)
+static bool virtio_net_load_ebpf(VirtIONet *n, Error **errp)
 {
     bool ret = false;
 
     if (virtio_net_attach_ebpf_to_backend(n->nic, -1)) {
-        if (!(n->ebpf_rss_fds && virtio_net_load_ebpf_fds(n))) {
-            ret = ebpf_rss_load(&n->ebpf_rss, NULL);
+        if (n->ebpf_rss_fds) {
+            ret = virtio_net_load_ebpf_fds(n, errp);
+        } else {
+            ret = ebpf_rss_load(&n->ebpf_rss, errp);
         }
     }
 
@@ -3761,7 +3762,23 @@ 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);
+        Error *err = NULL;
+        if (!virtio_net_load_ebpf(n, &err)) {
+            /*
+             * If user explicitly gave QEMU RSS FDs to use, then
+             * failing to use them must be considered a fatal
+             * error. If no RSS FDs were provided, QEMU is trying
+             * eBPF on a "best effort" basis only, so report a
+             * warning and allow fallback to software RSS.
+             */
+            if (n->ebpf_rss_fds) {
+                error_propagate(errp, err);
+            } else {
+                warn_report("unable to load eBPF RSS: %s",
+                            error_get_pretty(err));
+                error_free(err);
+            }
+        }
     }
 }
 
-- 
2.45.2

On 5/9/24 20:13, Daniel P. Berrangé wrote:
> If the user/mgmt app passed in a set of pre-opened FDs for eBPF RSS,
> then it is expecting QEMU to use them. Any failure to do so must be
> considered a fatal error and propagated back up the stack, otherwise
> deployment mistakes will not be detectable in a prompt manner. When
> not using pre-opened FDs, then eBPF RSS is tried on a "best effort"
> basis only and thus fallback to software RSS is valid.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>   hw/net/virtio-net.c | 41 +++++++++++++++++++++++++++++------------
>   1 file changed, 29 insertions(+), 12 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>