On Tue, Nov 18, 2025 at 1:35 AM Jon Kohler <[email protected]> wrote: > > > > > On Nov 16, 2025, at 11:32 PM, Jason Wang <[email protected]> wrote: > > > > On Fri, Nov 14, 2025 at 10:53 PM Jon Kohler <[email protected]> wrote: > >> > >> > >> > >>> On Nov 12, 2025, at 8:09 PM, Jason Wang <[email protected]> wrote: > >>> > >>> !-------------------------------------------------------------------| > >>> CAUTION: External Email > >>> > >>> |-------------------------------------------------------------------! > >>> > >>> On Thu, Nov 13, 2025 at 8:14 AM 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"). > >>> > >>> It has been used even before this commit. > >> > >> Ah, thanks for the pointer. I’d have to go dig to find its genesis, but > >> its more to say, this existed prior to the LFENCE commit. > >> > >>> > >>>> 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 > >>> > >>> I wonder if we need to test on archs like ARM. > >> > >> Are you thinking from a performance perspective? Or a correctness one? > > > > Performance, I think the patch is correct. > > > > Thanks > > > > Ok gotcha. If anyone has an ARM system stuffed in their > front pocket and can give this a poke, I’d appreciate it, as > I don’t have ready access to one personally. > > That said, I think this might end up in “well, it is what it is” > territory as Linus was alluding to, i.e. if performance dips on > ARM for vhost, then thats a compelling point to optimize whatever > ends up being the culprit for get/put user? > > Said another way, would ARM perf testing (or any other arch) be a > blocker to taking this change?
Not a must but at least we need to explain the implication for other archs as the discussion you quoted are all for x86. Thanks > > Thanks - Jon >
