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

Jason Wang
2025-11-20T01:57:26Z

On Tue, Nov 18, 2025 at 1:35 AM Jon Kohler <jon@nutanix.com> wrote:
>
>
>
> > On Nov 16, 2025, at 11:32 PM, Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Fri, Nov 14, 2025 at 10:53 PM Jon Kohler <jon@nutanix.com> wrote:
> >>
> >>
> >>
> >>> On Nov 12, 2025, at 8:09 PM, Jason Wang <jasowang@redhat.com> wrote:
> >>>
> >>> !-------------------------------------------------------------------|
> >>> CAUTION: External Email
> >>>
> >>> |-------------------------------------------------------------------!
> >>>
> >>> 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.
> >>
> >> Ah, thanks for the pointer. I’d have to go dig to find its genesis, but
> >> its more to say, this existed prior to the LFENCE 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.
> >>
> >> Are you thinking from a performance perspective? Or a correctness one?
> >
> > Performance, I think the patch is correct.
> >
> > Thanks
> >
>
> Ok gotcha. If anyone has an ARM system stuffed in their
> front pocket and can give this a poke, I’d appreciate it, as
> I don’t have ready access to one personally.
>
> That said, I think this might end up in “well, it is what it is”
> territory as Linus was alluding to, i.e. if performance dips on
> ARM for vhost, then thats a compelling point to optimize whatever
> ends up being the culprit for get/put user?
>
> Said another way, would ARM perf testing (or any other arch) be a
> blocker to taking this change?

Not a must but at least we need to explain the implication for other
archs as the discussion you quoted are all for x86.

Thanks

>
> Thanks - Jon
>