On Thu, Nov 27, 2025 at 03:11:57AM +0000, Jon Kohler wrote: > > > > On Nov 26, 2025, at 8:08 PM, Jason Wang <[email protected]> wrote: > > > > On Thu, Nov 27, 2025 at 3:48 AM Jon Kohler <[email protected]> wrote: > >> > >> > >>> On Nov 26, 2025, at 5:25 AM, Arnd Bergmann <[email protected]> wrote: > >>> > >>> On Wed, Nov 26, 2025, at 07:04, Jason Wang wrote: > >>>> 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: > >>>>> 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? > >>> > >>> I think the more relevant commit is for 64-bit Arm here, but this does > >>> the same thing, see 84624087dd7e ("arm64: uaccess: Don't bother > >>> eliding access_ok checks in __{get, put}_user"). > >> > >> Ah! Right, this is definitely the important bit, as it makes it > >> crystal clear that these are exactly the same thing. The current > >> code is: > >> #define get_user __get_user > >> #define put_user __put_user > >> > >> So, this patch changing from __* to regular versions is a no-op > >> on arm side of the house, yea? > >> > >>> I would think that if we change the __get_user() to get_user() > >>> in this driver, the same should be done for the > >>> __copy_{from,to}_user(), which similarly skips the access_ok() > >>> check but not the PAN/SMAP handling. > >> > >> Perhaps, thats a good call out. I’d file that under one battle > >> at a time. Let’s get get/put user dusted first, then go down > >> that road? > >> > >>> In general, the access_ok()/__get_user()/__copy_from_user() > >>> pattern isn't really helpful any more, as Linus already > >>> explained. I can't tell from the vhost driver code whether > >>> we can just drop the access_ok() here and use the plain > >>> get_user()/copy_from_user(), or if it makes sense to move > >>> to the newer user_access_begin()/unsafe_get_user()/ > >>> unsafe_copy_from_user()/user_access_end() and try optimize > >>> out a few PAN/SMAP flips in the process. > > > > Right, according to my testing in the past, PAN/SMAP is a killer for > > small packet performance (PPS). > > For sure, every little bit helps along that path > > > > >> > >> In general, I think there are a few spots where we might be > >> able to optimize (vhost_get_vq_desc perhaps?) as that gets > >> called quite a bit and IIRC there are at least two flips > >> in there that perhaps we could elide to one? An investigation > >> for another day I think. > > > > Did you mean trying to read descriptors in a batch, that would be > > better and with IN_ORDER it would be even faster as a single (at most > > two) copy_from_user() might work (without the need to use > > user_access_begin()/user_access_end(). > > Yep. I haven’t fully thought through it, just a drive-by idea > from looking at code for the recent work I’ve been doing, just > scratching my head thinking there *must* be something we can do > better there. > > Basically on the get rx/tx bufs path as well as the > vhost_add_used_and_signal_n path, I think we could cluster together > some of the get/put users and copy to/from’s. Would take some > massaging, but I think there is something there. > > >> > >> Anyhow, with this info - Jason - is there anything else you > >> can think of that we want to double click on? > > > > Nope. > > > > Thanks > > Ok thanks. Perhaps we can land this in next and let it soak in, > though, knock on wood, I don’t think there will be fallout > (famous last words!) ? >
To clairify, I think vhost tree is better to put this in next than net-next, both because it's purely core vhost and because unlike net-next vhost rebases so it is easy to just drop the patch if there are issues. I'll put it there. -- MST
