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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 14, 2025 at 07:30:32PM +0000, Jon Kohler wrote:
> 
> 
> > On Nov 14, 2025, at 1:54 PM, David Laight <david.laight.linux@gmail.com> wrote:
> > 
> > !-------------------------------------------------------------------|
> >  CAUTION: External Email
> > 
> > |-------------------------------------------------------------------!
> > 
> > On Wed, 12 Nov 2025 17:55:28 -0700
> > 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"). 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
> >> 
> >> As to what drives the speedup, Sean's patch [2] explains:
> >> Use the normal, checked versions for get_user() and put_user() instead of
> >> the double-underscore versions that omit range checks, as the checked
> >> versions are actually measurably faster on modern CPUs (12%+ on Intel,
> >> 25%+ on AMD).
> > 
> > Is there an associated access_ok() that can also be removed?
> > 
> > David
> 
> Hey David - IIUC, the access_ok() for non-iotlb setups is done at
> initial setup time, not per event, see vhost_vring_set_addr and
> for the vhost net side see vhost_net_set_backend -> 
> vhost_vq_access_ok.
> 
> Will lean on MST/Jason to help sanity check my understanding.

Right.

> In the iotlb case, that’s handled differently (Jason can speak to
> that side), but I dont think there is something we’d remove there?