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). > > 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(). > > Anyhow, with this info - Jason - is there anything else you > can think of that we want to double click on? Nope. Thanks > > Jon
