Re: [REGRESSION] Massive virtio-net throughput drop in guest VM with Linux 6.8+

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 02, 2025 at 11:12:07PM +0200, Markus Fohrer wrote:
> Hi,
> 
> I'm observing a significant performance regression in KVM guest VMs using virtio-net with recent Linux kernels (6.8.1+ and 6.14).
> 
> When running on a host system equipped with a Broadcom NetXtreme-E (bnxt_en) NIC and AMD EPYC CPUs, the network throughput in the guest drops to 100–200 KB/s. The same guest configuration performs normally (~100 MB/s) when using kernel 6.8.0 or when the VM is moved to a host with Intel NICs.
> 
> Test environment:
> - Host: QEMU/KVM, Linux 6.8.1 and 6.14.0
> - Guest: Linux with virtio-net interface
> - NIC: Broadcom BCM57416 (bnxt_en driver, no issues at host level)
> - CPU: AMD EPYC
> - Storage: virtio-scsi
> - VM network: virtio-net, virtio-scsi (no CPU or IO bottlenecks)
> - Traffic test: iperf3, scp, wget consistently slow in guest
> 
> This issue is not present:
> - On 6.8.0 
> - On hosts with Intel NICs (same VM config)
> 
> I have bisected the issue to the following upstream commit:
> 
>   49d14b54a527 ("virtio-net: Suppress tx timeout warning for small tx")
>   https://git.kernel.org/linus/49d14b54a527

Thanks a lot for the info!


both the link and commit point at:

commit 49d14b54a527289d09a9480f214b8c586322310a
Author: Eric Dumazet <edumazet@google.com>
Date:   Thu Sep 26 16:58:36 2024 +0000

    net: test for not too small csum_start in virtio_net_hdr_to_skb()
    

is this what you mean?

I don't know which commit is "virtio-net: Suppress tx timeout warning for small tx"



> Reverting this commit restores normal network performance in affected guest VMs.
> 
> I’m happy to provide more data or assist with testing a potential fix.
> 
> Thanks,
> Markus Fohrer


Thanks! First I think it's worth checking what is the setup, e.g.
which offloads are enabled.
Besides that, I'd start by seeing what's doing on. Assuming I'm right about
Eric's patch:

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 276ca543ef44d8..02a9f4dc594d02 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -103,8 +103,10 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 
 		if (!skb_partial_csum_set(skb, start, off))
 			return -EINVAL;
+		if (skb_transport_offset(skb) < nh_min_len)
+			return -EINVAL;
 
-		nh_min_len = max_t(u32, nh_min_len, skb_transport_offset(skb));
+		nh_min_len = skb_transport_offset(skb);
 		p_off = nh_min_len + thlen;
 		if (!pskb_may_pull(skb, p_off))
 			return -EINVAL;


sticking a printk before return -EINVAL to show the offset and nh_min_len
would be a good 1st step. Thanks!