On Thu, Nov 13, 2025 at 8:14 AM Jon Kohler [off-list ref] wrote:

vhost_get_user and vhost_put_user leverage __get_user and __put_user,
respectively, which were both added in 2016 by commit 6b1e6cc7855b
("vhost: new device IOTLB API").

It has been used even before this commit.

In a heavy UDP transmit workload on a
vhost-net backed tap device, these functions showed up as ~11.6% of
samples in a flamegraph of the underlying vhost worker thread.

Quoting Linus from [1]:
    Anyway, every single __get_user() call I looked at looked like
    historical garbage. [...] End result: I get the feeling that we
    should just do a global search-and-replace of the __get_user/
    __put_user users, replace them with plain get_user/put_user instead,
    and then fix up any fallout (eg the coco code).

Switch to plain get_user/put_user in vhost, which results in a slight
throughput speedup. get_user now about ~8.4% of samples in flamegraph.

Basic iperf3 test on a Intel 5416S CPU with Ubuntu 25.10 guest:
TX: taskset -c 2 iperf3 -c <rx_ip> -t 60 -p 5200 -b 0 -u -i 5
RX: taskset -c 2 iperf3 -s -p 5200 -D
Before: 6.08 Gbits/sec
After:  6.32 Gbits/sec

I wonder if we need to test on archs like ARM.

Thanks