Re: [PATCH net-next] vhost: use "checked" versions of get_user() and put_user()

Jason Wang
2025-11-27T01:09:10Z

On Thu, Nov 27, 2025 at 3:48 AM Jon Kohler <jon@nutanix.com> wrote:
>
>
>
> > On Nov 26, 2025, at 5:25 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > On Wed, Nov 26, 2025, at 07:04, Jason Wang wrote:
> >> On Wed, Nov 26, 2025 at 3:45 AM Jon Kohler <jon@nutanix.com> wrote:
> >>>> On Nov 19, 2025, at 8:57 PM, Jason Wang <jasowang@redhat.com> wrote:
> >>>> On Tue, Nov 18, 2025 at 1:35 AM Jon Kohler <jon@nutanix.com> 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