[libvirt-users] Need to increase the rx and tx buffer size of my interface

[Ashish Kurian]

Dear Users/Developers,

I am using a KVM Ubuntu VM as a degrader to apply specific delays to
incoming packets. As the delay for my packets can be higher than 7.5
seconds, there is not enough buffer on my interface to buffer all the
packets. Therefore those overflowing packets are dropped in the machine and
not forwarded.

When I tried to use the command  ethtool -G ens8 rx 512 to increase the
buffer size, I get the following error.

Cannot set device ring parameters: Operation not permitted

I have kept the VM xml files as specified in the link :
https://libvirt.org/formatdomain.html. The value that I kept in my xml file
is as follows.

<interface type='direct'>
      <mac address='52:54:00:72:f9:eb'/>
      <source dev='enp7s0f0' mode='vepa'/>
      <model type='virtio'/>
      <driver name='vhost' queues='5' rx_queue_size='512'
tx_queue_size='512'>
      <host csum='off' gso='off' tso4='off' tso6='off' ecn='off' ufo='off'
mrg_rxbuf='off'/>
      <guest csum='off' tso4='off' tso6='off' ecn='off' ufo='off'/>
      </driver>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08'
function='0x0'/>
    </interface>
    <interface type='direct'>
      <mac address='52:54:00:00:b5:99'/>
      <source dev='enp7s0f1' mode='vepa'/>
      <model type='virtio'/>
      <driver name='vhost' queues='5' rx_queue_size='512'
tx_queue_size='512'>
      <host csum='off' gso='off' tso4='off' tso6='off' ecn='off' ufo='off'
mrg_rxbuf='off'/>
      <guest csum='off' tso4='off' tso6='off' ecn='off' ufo='off'/>
      </driver>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09'
function='0x0'/>

After I saved the xml and redefined the machine and started it up, I
checked to see if the buffers are increased. I still see that the buffers
are as follows.

Ring parameters for ens9:
Pre-set maximums:
RX: 4096
RX Mini: 0
RX Jumbo: 0
TX: 4096
Current hardware settings:
RX: *256*
RX Mini: 0
RX Jumbo: 0
TX: *256*


How can I increase the buffer on my KVM VM?

Best Regards,
Ashish Kurian
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