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

[Markus Fohrer <markus.fohrer@webked.de>]

Am Donnerstag, dem 03.04.2025 um 10:03 -0400 schrieb Michael S.
Tsirkin:
> On Thu, Apr 03, 2025 at 03:51:01PM +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@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!
> > > 
> > 
> > 
> > Hi Eric,
> > 
> > thanks a lot for the quick response — and yes, you're absolutely
> > right.
> > 
> > Apologies for the confusion: I mistakenly wrote the wrong commit
> > description in my initial mail.
> > 
> > The correct commit is indeed:
> > 
> > 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()
> > 
> > This is the one I bisected and which causes the performance
> > regression
> > in my environment.
> > 
> > Thanks again,
> > Markus
> 
> 
> I'm not Eric but good to know.
> Alright, so I would start with the two items: device features and
> printk.
> 

as requested, here’s the device/feature information from the guest
running kernel 6.14 (mainline):

Interface: ens18

ethtool -i ens18:
driver: virtio_net
version: 1.0.0
firmware-version: 
expansion-rom-version: 
bus-info: 0000:00:12.0
supports-statistics: yes
supports-test: no
supports-eeprom-access: no
supports-register-dump: no
supports-priv-flags: no


ethtool -k ens18:
Features for ens18:
rx-checksumming: on [fixed]
tx-checksumming: on
	tx-checksum-ipv4: off [fixed]
	tx-checksum-ip-generic: on
	tx-checksum-ipv6: off [fixed]
	tx-checksum-fcoe-crc: off [fixed]
	tx-checksum-sctp: off [fixed]
scatter-gather: on
	tx-scatter-gather: on
	tx-scatter-gather-fraglist: off [fixed]
tcp-segmentation-offload: on
	tx-tcp-segmentation: on
	tx-tcp-ecn-segmentation: on
	tx-tcp-mangleid-segmentation: off
	tx-tcp6-segmentation: on
generic-segmentation-offload: on
generic-receive-offload: on
large-receive-offload: off [fixed]
rx-vlan-offload: off [fixed]
tx-vlan-offload: off [fixed]
ntuple-filters: off [fixed]
receive-hashing: off [fixed]
highdma: on [fixed]
rx-vlan-filter: on [fixed]
vlan-challenged: off [fixed]
tx-gso-robust: on [fixed]
tx-fcoe-segmentation: off [fixed]
tx-gre-segmentation: off [fixed]
tx-gre-csum-segmentation: off [fixed]
tx-ipxip4-segmentation: off [fixed]
tx-ipxip6-segmentation: off [fixed]
tx-udp_tnl-segmentation: off [fixed]
tx-udp_tnl-csum-segmentation: off [fixed]
tx-gso-partial: off [fixed]
tx-tunnel-remcsum-segmentation: off [fixed]
tx-sctp-segmentation: off [fixed]
tx-esp-segmentation: off [fixed]
tx-udp-segmentation: off
tx-gso-list: off [fixed]
tx-nocache-copy: off
loopback: off [fixed]
rx-fcs: off [fixed]
rx-all: off [fixed]
tx-vlan-stag-hw-insert: off [fixed]
rx-vlan-stag-hw-parse: off [fixed]
rx-vlan-stag-filter: off [fixed]
l2-fwd-offload: off [fixed]
hw-tc-offload: off [fixed]
esp-hw-offload: off [fixed]
esp-tx-csum-hw-offload: off [fixed]
rx-udp_tunnel-port-offload: off [fixed]
tls-hw-tx-offload: off [fixed]
tls-hw-rx-offload: off [fixed]
rx-gro-hw: on
tls-hw-record: off [fixed]
rx-gro-list: off
macsec-hw-offload: off [fixed]
rx-udp-gro-forwarding: off
hsr-tag-ins-offload: off [fixed]
hsr-tag-rm-offload: off [fixed]
hsr-fwd-offload: off [fixed]
hsr-dup-offload: off [fixed]

ethtool ens18:
Settings for ens18:
	Supported ports: [  ]
	Supported link modes:   Not reported
	Supported pause frame use: No
	Supports auto-negotiation: No
	Supported FEC modes: Not reported
	Advertised link modes:  Not reported
	Advertised pause frame use: No
	Advertised auto-negotiation: No
	Advertised FEC modes: Not reported
	Speed: Unknown!
	Duplex: Unknown! (255)
	Auto-negotiation: off
	Port: Other
	PHYAD: 0
	Transceiver: internal
netlink error: Operation not permitted
	Link detected: yes


Kernel log (journalctl -k):
Apr 03 19:50:37 kb-test.allod.com kernel: virtio_scsi virtio2: 4/0/0
default/read/poll queues  
Apr 03 19:50:37 kb-test.allod.com kernel: virtio_net virtio1 ens18:
renamed from eth0

Let me know if you’d like comparison data from kernel 6.11 or any
additional tests