Re: [PATCH next 0/3] iov: Optimise user copies

[David Laight]

On Fri, 21 Mar 2025 16:35:52 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Fri, 21 Mar 2025 at 15:46, David Laight <david.laight.linux@xxxxxxxxx> wrote:
> >
> > The speculation barrier in access_ok() is expensive.
> >
> > The first patch removes the initial checks when reading the iovec[].
> > The checks are repeated before the actual copy.
> >
> > The second patch uses 'user address masking' if supported.
> >
> > The third removes a lot of code for single entry iovec[].  
> 
> Ack, except I'd really like to see numbers for things that claim to
> remove expensive stuff.

Except that some of the 'expensive stuff' is missing!

copy_from_user_iter() does:
	if (access_ok())
		raw_copy_from_user();
So it is missing the barrier_nospec().
The error handling is also different from _inline_copy_from_user().
(It doesn't zero-fill after a partial read.)

The observant will also notice that it is missing the massive
performance hit (and code bloat) of check_copy_size() (usercopy hardening).

Talking of performance I've dug out my clock cycle measuring code
(still full of different ipcsum functions).
I'm sure I got 12 bytes/clock on my i7-7 for the loop in the current kernel,
but it is only giving 10 today (possibly I don't have the latest version).
OTOH my new zen5 runs the adxo/adxc loop at 16 bytes/clock (i7-7 manages 12).
I'm going to try to find time for some memcpy() experiments.

	David

> 
> But yeah, the patches look sane.
> 
>           Linus