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FreeBSD-EN-16:07.ipi                                            Errata Notice
                                                          The FreeBSD Project

Topic:          Excessive latency in x86 IPI delivery

Category:       core
Module:         kernel
Announced:      2016-05-04
Credits:        Stanislav Sedov
Affects:        FreeBSD 10.2 and 10.3
Corrected:      2016-04-27 19:12:49 UTC (stable/10, 10.3-STABLE)
                2016-05-04 15:25:47 UTC (releng/10.3, 10.3-RELEASE-p2)
                2016-05-04 15:26:23 UTC (releng/10.2, 10.2-RELEASE-p16)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security branches,
and the following sections, please visit
<URL:https://security.freebsd.org/>.

I.   Background

Each CPU in an x86 system contains an interrupt controller (local APIC)
used to handle both external interrupts and inter-processor-interrupts
(IPIs).

II.  Problem Description

In xAPIC mode, the local APIC can only queue a single IPI at a time.  If
a previously queued IPI is still pending when a CPU attempts to send an
IPI, it spins waiting for the previous IPI to complete.  A change merged
prior to 10.2-RELEASE altered this loop to check the state of the previous
IPI once every 5 microseconds rather than constantly polling.

III. Impact

Checking the status of the previous IPI once every 5 microseconds could
introduce 5 microsecond delays when sending IPIs.  This could increase the
latency of various scheduling operations reducing performance.

IV.  Workaround

No workaround is available, but non-x86 systems are not affected.

V.   Solution

Perform one of the following:

1) Upgrade your system to a supported FreeBSD stable or release / security
branch (releng) dated after the correction date.

2) To update your present system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

3) To update your present system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/EN-16:07/ipi.patch
# fetch https://security.FreeBSD.org/patches/EN-16:07/ipi.patch.asc
# gpg --verify ipi.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r298715
releng/10.2/                                                      r299067
releng/10.3/                                                      r299066
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

The latest revision of this Errata Notice is available at
https://security.FreeBSD.org/advisories/FreeBSD-EN-16:07.ipi.asc
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