Messages in this thread | | | Date | Tue, 25 Jul 2017 02:37:57 +0100 | From | Al Viro <> | Subject | Re: Simplfying copy_siginfo_to_user |
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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 > <ebiederm@xmission.com> wrote: > > 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...
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