> On Nov 14, 2025, at 1:54 PM, David Laight <[email protected]> > wrote: > > !-------------------------------------------------------------------| > CAUTION: External Email > > |-------------------------------------------------------------------! > > On Wed, 12 Nov 2025 17:55:28 -0700 > Jon Kohler <[email protected]> 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. 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?
