Re: How to check tap device for sndbuf when I set sndbuf=0 in the interface xml

[Yalan Zhang <yalzhang@xxxxxxxxxx>]

cc libvirt-users for more inputs

Hi, 

I'm trying to find out how to check sndbuf for a tap device, could you please help to check it? 

In kernel 2.6.18, the tap device's default sndbuf is 1MB. There is a RFE bug[1] and patch[2] to introduce the option below in libvirt to adjust the sndbuf.

<tune> 

<sndbuf>1600</sndbuf> 

</tune>

It is said in the patch[2] that when we set sndbuf=0, we actually set it to 0xffffffff.

How to check if it is set successfully? I have checked the below files, the value didn't change after I start a vm with sndbuf=0

# grep . /proc/sys/net/core/*mem_default
/proc/sys/net/core/rmem_default:212992
/proc/sys/net/core/wmem_default:212992

And from the doc[3]: "The default value is set by the /proc/sys/net/core/wmem_default file and the maximum allowed value is set by the /proc/sys/net/core/wmem_max file."

Current kernel 5.14.0-177.el9 has some updates about sndbuf:

# cat  /proc/sys/net/core/wmem_default
212992

# cat  /proc/sys/net/core/wmem_max
212992

Thank you!

[1] Bug 665293 - RFE: Allow setting size of send buffer per TAP device in QEMU driver

[2] https://listman.redhat.com/archives/libvir-list/2011-January/032763.html

[3] https://man7.org/linux/man-pages/man7/socket.7.html

Yalan