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vhost_get_user and vhost_put_user leverage __get_user and __put_user, respectively, which were both added in 2016 by commit6b1e6cc785("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). The performance hit on the unchecked versions is almost entirely due to the added LFENCE on CPUs where LFENCE is serializing (which is effectively all modern CPUs), which was added by commit304ec1b050("x86/uaccess: Use __uaccess_begin_nospec() and uaccess_try_nospec"). The small optimizations done by commitb19b74bc99("x86/mm: Rework address range check in get_user() and put_user()") likely shave a few cycles off, but the bulk of the extra latency comes from the LFENCE. [1] https://lore.kernel.org/all/CAHk-=wiJiDSPZJTV7z3Q-u4DfLgQTNWqUqqrwSBHp0+Dh016FA@mail.gmail.com/ [2] https://lore.kernel.org/all/20251106210206.221558-1-seanjc@google.com/ Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Sean Christopherson <seanjc@google.com> Signed-off-by: Jon Kohler <jon@nutanix.com> Message-Id: <20251113005529.2494066-1-jon@nutanix.com> Acked-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>