On Wed, Nov 26, 2025 at 3:45 AM Jon Kohler <[email protected]> wrote: > > > > > On Nov 19, 2025, at 8:57 PM, Jason Wang <[email protected]> wrote: > > > > 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: > >>>>> > >>>>> 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 > > I’ll admit my ARM muscle is weak, but here’s my best take on this: > > Looking at arch/arm/include/asm/uaccess.h, the biggest thing that I > noticed is the CONFIG_CPU_SPECTRE ifdef, which already remaps > __get_user() to get_user(), so anyone running that in their kconfig > will already practically have the behavior implemented by this patch > by way of commit b1cd0a148063 ("ARM: spectre-v1: use get_user() for > __get_user()”). > > Same deal goes for __put_user() vs put_user by way of commit > e3aa6243434f ("ARM: 8795/1: spectre-v1.1: use put_user() for __put_user()”) > > Looking at arch/arm/mm/Kconfig, there are a variety of scenarios > where CONFIG_CPU_SPECTRE will be enabled automagically. Looking at > commit 252309adc81f ("ARM: Make CONFIG_CPU_V7 valid for 32bit ARMv8 > implementations") > it says that "ARMv8 is a superset of ARMv7", so I’d guess that just > about everything ARM would include this by default? > > If so, that mean at least for a non-zero population of ARM’ers, > they wouldn’t notice anything from this patch, yea?
Adding ARM maintainers for more thought. Thanks > > Happy to learn otherwise if that read is incorrect! > > Thanks all, > Jon
