Re: Simplfying copy_siginfo_to_user

On Mon, Jul 24, 2017 at 10:43:34AM -0700, Linus Torvalds wrote:

On Sat, Jul 22, 2017 at 1:25 PM, Eric W. Biederman
[off-list ref] wrote:

quoted

I played with some clever changes such as limiting the copy to 48 bytes,
disabling the memset and the like but I could not get a strong enough
signal to say that any one change removed the extra or a clear part of
it 20ns.

What CPU did you use? Because the SMAP bit in particular matters.

The field-by-field copies are extremely slow on modern CPU's that
implement SMAP, unless you also use the special "unsafe_put_user()"
code (or the nasty old put_user_ex() code that some of the x86 signal
code uses).

So one of the advantages of just copy_to_user() ends up being visible
only on Broadwell+ (or whatever the SMAP cutoff is).

Guys, could you take a look at vfs.git#work.siginfo?  I'd been pretty
much buried lately (and probably will for several more weeks - long-distance
moves *suck*), so that thing got stalled, but it might be worth a look.

The code generated in copy_siginfo_to_user() in it looks reasonably good,
we don't copy more than we need and all copying to userland is done
by copy_to_user() - one call per call of copy_siginfo_to_user(), so
SMAP crap is not an issue.

The next thing I hope to do is converting compat side of that thing to
the same; that got stalled.

Al "Buried in boxes" Viro...