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

Jason Wang
2025-11-26T06:04:39Z

On Wed, Nov 26, 2025 at 3:45 AM Jon Kohler <jon@nutanix.com> wrote:
>
>
>
> > On Nov 19, 2025, at 8:57 PM, Jason Wang <jasowang@redhat.com> wrote:
> >
> > 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:
> >>>>>
> >>>>> 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
>
> I’ll admit my ARM muscle is weak, but here’s my best take on this:
>
> Looking at arch/arm/include/asm/uaccess.h, the biggest thing that I
> noticed is the CONFIG_CPU_SPECTRE ifdef, which already remaps
> __get_user() to get_user(), so anyone running that in their kconfig
> will already practically have the behavior implemented by this patch
> by way of commit b1cd0a148063 ("ARM: spectre-v1: use get_user() for
> __get_user()”).
>
> Same deal goes for __put_user() vs put_user by way of commit
> e3aa6243434f ("ARM: 8795/1: spectre-v1.1: use put_user() for __put_user()”)
>
> Looking at arch/arm/mm/Kconfig, there are a variety of scenarios
> where CONFIG_CPU_SPECTRE will be enabled automagically. Looking at
> commit 252309adc81f ("ARM: Make CONFIG_CPU_V7 valid for 32bit ARMv8 implementations")
> it says that "ARMv8 is a superset of ARMv7", so I’d guess that just
> about everything ARM would include this by default?
>
> If so, that mean at least for a non-zero population of ARM’ers,
> they wouldn’t notice anything from this patch, yea?

Adding ARM maintainers for more thought.

Thanks

>
> Happy to learn otherwise if that read is incorrect!
>
> Thanks all,
> Jon