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

[Michael S. Tsirkin]

On Fri, Apr 04, 2025 at 10:16:55AM +0200, Markus Fohrer wrote:
> Am Donnerstag, dem 03.04.2025 um 09:04 -0400 schrieb Michael S.
> Tsirkin:
> > 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@xxxxxxxxxx>
> > 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!
> > 
> 
> I added the following printk inside virtio_net_hdr_to_skb():
> 
>     if (skb_transport_offset(skb) < nh_min_len){
>         printk(KERN_INFO "virtio_net: 3 drop, transport_offset=%u,
> nh_min_len=%u\n",
>                skb_transport_offset(skb), nh_min_len);
>         return -EINVAL;
>     }
> 
> Built and installed the kernel, then triggered a large download via:
> 
>     wget http://speedtest.belwue.net/10G
> 
> Relevant output from `dmesg -w`:
> 
> [   57.327943] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.428942] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.428962] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.553068] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.553088] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.576678] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.618438] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.618453] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.703077] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.823072] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.891982] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   57.946190] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  
> [   58.218686] virtio_net: 3 drop, transport_offset=34, nh_min_len=40  

Hmm indeed. And what about these values?
                u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
                u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
                u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
print them too?



> I would now do the test with commit
> 49d14b54a527289d09a9480f214b8c586322310a and commit
> 49d14b54a527289d09a9480f214b8c586322310a~1
> 

Worth checking though it seems likely now the hypervisor is doing weird
things. what kind of backend is it? qemu? tun? vhost-user? vhost-net?

-- 
MST