On Fri, Nov 14, 2025 at 07:30:32PM +0000, Jon Kohler wrote:

quoted

On Nov 14, 2025, at 1:54 PM, David Laight [off-list ref] wrote:

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On Wed, 12 Nov 2025 17:55:28 -0700
Jon Kohler [off-list ref] wrote:

quoted

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?