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 -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 commit 304ec1b05031 ("x86/uaccess: Use __uaccess_begin_nospec() and uaccess_try_nospec"). The small optimizations done by commit b19b74bc99b1 ("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 Cc: Borislav Petkov Cc: Sean Christopherson Signed-off-by: Jon Kohler --- drivers/vhost/vhost.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 8570fdf2e14a..ffbd0a9a7a03 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -1442,13 +1442,13 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq, ({ \ int ret; \ if (!vq->iotlb) { \ - ret = __put_user(x, ptr); \ + ret = put_user(x, ptr); \ } else { \ __typeof__(ptr) to = \ (__typeof__(ptr)) __vhost_get_user(vq, ptr, \ sizeof(*ptr), VHOST_ADDR_USED); \ if (to != NULL) \ - ret = __put_user(x, to); \ + ret = put_user(x, to); \ else \ ret = -EFAULT; \ } \ @@ -1487,14 +1487,14 @@ static inline int vhost_put_used_idx(struct vhost_virtqueue *vq) ({ \ int ret; \ if (!vq->iotlb) { \ - ret = __get_user(x, ptr); \ + ret = get_user(x, ptr); \ } else { \ __typeof__(ptr) from = \ (__typeof__(ptr)) __vhost_get_user(vq, ptr, \ sizeof(*ptr), \ type); \ if (from != NULL) \ - ret = __get_user(x, from); \ + ret = get_user(x, from); \ else \ ret = -EFAULT; \ } \ -- 2.43.0