Re: [PATCH net-next] vhost: use "checked" versions of get_user() and put_user()

Jason Wang
2025-11-13T01:10:12Z

On Thu, Nov 13, 2025 at 8:14 AM Jon Kohler <jon@nutanix.com> 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