Question about the sndbuf of the tap interface with vhost-net

[Harold Huang]

I see in dpdk virtio-user driver, the TUNSETSNDBUF is initialized with
INT_MAX, see: https://github.com/DPDK/dpdk/blob/main/drivers/net/virtio/virtio_user/vhost_kernel_tap.c#L169
It is ok because tap driver uses it to support tx baching, see this
patch: https://github.com/torvalds/linux/commit/0a0be13b8fe2cac11da2063fb03f0f39359b3069

But in tun_xdp_one, napi is not supported and I want to user napi in
tun_get_user to enable gro. As I result, I change the sndbuf to a
value such as 212992 in /proc/sys/net/core/wmem_default. But the
performance tested by iperf is greatly degraded, from 4.5 Gbps to
750Gbps per flow. I see the the iperf server consume 100% cpu core,
which should be the bottleneck of the this test. The perf top result
of iperf server cpu core is as follows:

'''
Samples: 72  of event 'cycles', 4000 Hz, Event count (approx.):
22685278 lost: 0/0 drop: 0/0
Overhead  Shared O  Symbol
  59.86%  [kernel]  [k] report_bug
  20.66%  [kernel]  [k] module_find_bug
   6.51%  [kernel]  [k] common_interrupt
   2.82%  [kernel]  [k] __slab_free
   1.48%  [kernel]  [k] copy_user_enhanced_fast_string
   1.44%  [kernel]  [k] __skb_datagram_iter
   1.42%  [kernel]  [k] notifier_call_chain
   1.41%  [kernel]  [k] irq_work_run_list
   1.41%  [kernel]  [k] update_irq_load_avg
   1.41%  [kernel]  [k] task_tick_fair
   1.41%  [kernel]  [k] cmp_ex_search
   0.16%  [kernel]  [k] __ghes_peek_estatus.isra.12
   0.02%  [kernel]  [k] acpi_os_read_memory
   0.00%  [kernel]  [k] native_apic_mem_write
'''
I am not clear about the test result. Can we change the sndbuf size in
dpdk? Is any way to enable vhost_net to use napi without changing the
tun kernel driver?