What impacts timestamping capabilities of a network interface?

Question

I have two Linux virtual machines in VirtualBox 7.0.

• Machine 1: CentOS 6 with Linux kernel 2.6.32-754
• Machine 2: CentOS 8 with Linux kernel 4.18.0-448

All network interfaces of machine 1 do not support TX timestamping, while all network interfaces of machine 2 support this feature.

Let's consider loopback interface.

Output of sudo ethtool -T lo

Machine 1:

Time stamping parameters for lo:
Capabilities:
        software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
        software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none

Machine 2:

Time stamping parameters for lo:
Capabilities:
        software-transmit
        software-receive
        software-system-clock
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none

It is not clear to me why machine 1 lacks support of TX timestamping, as this feature should be available for this kernel: documentation

I am on purpose checking loopback interface here as I expect it should not depend on VirtualBox implementation details.

Does anyone know if something should be explicitly configured in the system to enable TX timestamps for CentOS6 (in fact, for any OS)?

Answer 1

I think that it has to do with how the drivers are written. For example, kernel sources v4.18 contain driver for loopback interface where there is such code:

static int loopback_get_ts_info(struct net_device *netdev,
                struct ethtool_ts_info *ts_info)
{
    ts_info->so_timestamping = SOF_TIMESTAMPING_TX_SOFTWARE |
                   SOF_TIMESTAMPING_RX_SOFTWARE |
                   SOF_TIMESTAMPING_SOFTWARE;

    ts_info->phc_index = -1;

    return 0;
};

So it means that loopback interface driver in this version supports TX timestamping.

But if we switch kernel version to e.g. v3.10 then it is clear that loopback driver does not support TX timestamping (SOF_TIMESTAMPING_TX_SOFTWARE).