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Re: Simplfying copy_siginfo_to_user

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Linus Torvalds <torvalds@linux-foundation.org> writes:

> 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).

Good point.

The cpu I was testing on was an AMD A10.  I don't actually have a cpu
that supports SMAP handy.

If you would like I can post the minimal patches and benckmark so anyone
who is interested could reproduce this for themselves.

I suspect that if it is down to only 20ns without SMAP this will
definitely be a performance improvement in the presence of SMAP.

Eric

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[PATCH v2 0/7] signal: Fix sending signals with siginfo ebiederm@xmission.com (Eric W. Biederman) - 2017-07-18 16:20 +0200
  [PATCH 6/7] fcntl: Don't use ambiguous SIG_POLL si_codes "Eric W. Biederman" <ebiederm@xmission.com> - 2017-07-18 16:20 +0200
    Re: [PATCH 6/7] fcntl: Don't use ambiguous SIG_POLL si_codes Oleg Nesterov <oleg@redhat.com> - 2017-07-20 18:20 +0200
      Re: [PATCH 6/7] fcntl: Don't use ambiguous SIG_POLL si_codes ebiederm@xmission.com (Eric W. Biederman) - 2017-07-21 04:50 +0200
  [PATCH 4/7] signal/mips: Document a conflict with SI_USER with SIGFPE "Eric W. Biederman" <ebiederm@xmission.com> - 2017-07-18 16:20 +0200
  [PATCH 3/7] signal/sparc: Document a conflict with SI_USER with SIGFPE "Eric W. Biederman" <ebiederm@xmission.com> - 2017-07-18 16:20 +0200
  [PATCH 1/7] signal/alpha: Document a conflict with SI_USER for SIGTRAP "Eric W. Biederman" <ebiederm@xmission.com> - 2017-07-18 16:20 +0200
    Re: [PATCH 1/7] signal/alpha: Document a conflict with SI_USER for  SIGTRAP Richard Henderson <rth@twiddle.net> - 2017-07-18 20:30 +0200
  Re: [PATCH 7/7] signal: Remove kernel interal si_code magic Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-18 19:00 +0200
    Re: [PATCH 7/7] signal: Remove kernel interal si_code magic ebiederm@xmission.com (Eric W. Biederman) - 2017-07-18 19:40 +0200
      Simplfying copy_siginfo_to_user ebiederm@xmission.com (Eric W. Biederman) - 2017-07-22 22:40 +0200
        Re: Simplfying copy_siginfo_to_user Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-24 19:50 +0200
          Re: Simplfying copy_siginfo_to_user ebiederm@xmission.com (Eric W. Biederman) - 2017-07-24 21:20 +0200
          Re: Simplfying copy_siginfo_to_user Al Viro <viro@ZenIV.linux.org.uk> - 2017-07-25 03:40 +0200

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