diff --git a/tools/tools/git/Makefile b/tools/tools/git/Makefile
new file mode 100644
--- /dev/null
+++ b/tools/tools/git/Makefile
@@ -0,0 +1,4 @@
+SCRIPTS=	git-arc.sh
+BINDIR?=	/usr/local/bin
+
+.include <bsd.prog.mk>
diff --git a/tools/tools/git/git-arc.sh b/tools/tools/git/git-arc.sh
new file mode 100644
--- /dev/null
+++ b/tools/tools/git/git-arc.sh
@@ -0,0 +1,596 @@
+#!/bin/sh
+#
+# SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+#
+# Copyright (c) 2019-2021 Mark Johnston <markj@FreeBSD.org>
+# Copyright (c) 2021 John Baldwin <jhb@FreeBSD.org>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in
+#    the documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+#
+
+# TODO:
+# - roll back after errors or SIGINT
+#   - current checkout
+#   - created revs
+#   - main (for git arc stage)
+
+warn()
+{
+    echo "$(basename $0): $1" >&2
+}
+
+err()
+{
+    warn "$1"
+    exit 1
+}
+
+err_usage()
+{
+    cat >&2 <<__EOF__
+Usage: git arc [-vy] <command> <arguments>
+
+Commands:
+  create [-l] [-r <reviewer1>[,<reviewer2>...]] [-s subscriber[,...]] [<commit>|<commit range>]
+  list <commit>|<commit range>
+  patch <diff1> [<diff2> ...]
+  stage [-b branch] [<commit>|<commit range>]
+  update <commit>
+
+Description:
+  Create or manage FreeBSD Phabricator reviews based on git commits.  There
+  is a one-to one relationship between git commits and Differential revisions,
+  and the Differential revision title must match the summary line of the
+  corresponding commit.  In particular, commit summaries must be unique across
+  all open Differential revisions authored by you.
+
+  The first parameter must be a verb.  The available verbs are:
+
+    create -- Create new Differential revisions from the specified commits.
+    list   -- Print the associated Differential revisions for the specified
+              commits.
+    patch  -- Try to apply a patch from a Differential revision to the
+              currently checked out tree.
+    stage  -- Prepare a series of commits to be pushed to the upstream FreeBSD
+              repository.  The commits are cherry-picked to a branch (main by
+              default), review tags are added to the commit log message, and
+              the log message is opened in an editor for any last-minute
+              updates.  The commits need not have associated Differential
+              revisions.
+    update -- Synchronize the Differential revisions associated with the
+              specified commits.  Currently only the diff is updated; the
+              review description and other metadata is not synchronized.
+
+  The typical end-to-end usage looks something like this:
+
+    $ git commit -m "kern: Rewrite in Rust"
+    $ git arc create HEAD
+    <Make changes to the diff based on reviewer feedback.>
+    $ git commit --amend
+    $ git arc update HEAD
+    <Now that all reviewers are happy, it's time to push.>
+    $ git arc stage HEAD
+    $ git push freebsd HEAD:main
+
+Config Variables:
+  These are manipulated by git-config(1).
+
+    arc.assume_yes [bool]
+                       -- Assume a "yes" answer to all prompts instead of
+                          prompting the user.  Equivalent to the -y flag.
+
+    arc.browse [bool]  -- Try to open newly created reviews in a browser tab.
+                          Defaults to false.
+
+    arc.list [bool]    -- Always use "list mode" (-l) with create.  In this
+                          mode, the list of git revisions to create reviews for
+                          is listed with a single prompt before creating
+                          reviews.  The diffs for individual commits are not
+                          shown.
+
+    arc.verbose [bool] -- Verbose output.  Equivalent to the -v flag.
+
+Examples:
+  Create a Phabricator review using the contents of the most recent commit in
+  your git checkout.  The commit title is used as the review title, the commit
+  log message is used as the review description, markj@FreeBSD.org is added as
+  a reviewer.
+
+  $ git arc create -r markj HEAD
+
+  Create a series of Phabricator reviews for each of HEAD~2, HEAD~ and HEAD.
+  Pairs of consecutive commits are linked into a patch stack.  Note that the
+  first commit in the specified range is excluded.
+
+  $ git arc create HEAD~3..HEAD
+
+  Update the review corresponding to commit b409afcfedcdda.  The title of the
+  commit must be the same as it was when the review was created.  The review
+  description is not automatically updated.
+
+  $ git arc update b409afcfedcdda
+
+  Apply the patch in review D12345 to the currently checked-out tree, and stage
+  it.
+
+  $ git arc patch D12345
+
+  List the status of reviews for all the commits in the branch "feature":
+
+  $ git arc list main..feature
+
+__EOF__
+
+    exit 1
+}
+
+diff2phid()
+{
+    local diff
+
+    diff=$1
+    if ! expr "$diff" : 'D[1-9][0-9]*$' >/dev/null; then
+        err "invalid diff ID $diff"
+    fi
+
+    echo '{"names":["'$diff'"]}' |
+        arc call-conduit -- phid.lookup |
+        jq -r "select(.response != []) | .response.${diff}.phid"
+}
+
+diff2status()
+{
+    local diff tmp status summary
+
+    diff=$1
+    if ! expr "$diff" : 'D[1-9][0-9]*$' >/dev/null; then
+        err "invalid diff ID $diff"
+    fi
+
+    tmp=$(mktemp)
+    echo '{"names":["'$diff'"]}' |
+        arc call-conduit -- phid.lookup > $tmp
+    status=$(jq -r "select(.response != []) | .response.${diff}.status" < $tmp)
+    summary=$(jq -r "select(.response != []) |
+         .response.${diff}.fullName" < $tmp)
+    printf "%-14s %s\n" "${status}" "${summary}"
+}
+
+log2diff()
+{
+    local diff
+
+    diff=$(git show -s --format=%B $commit |
+        sed -nE '/^Differential Revision:[[:space:]]+(https:\/\/reviews.freebsd.org\/)?(D[0-9]+)$/{s//\2/;p;}')
+    if [ -n "$diff" ] && [ $(echo "$diff" | wc -l) -eq 1 ]; then
+        echo $diff
+    else
+        echo
+    fi
+}
+
+commit2diff()
+{
+    local commit diff title
+
+    commit=$1
+
+    # First, look for a valid differential reference in the commit
+    # log.
+    diff=$(log2diff $commit)
+    if [ -n "$diff" ]; then
+        echo $diff
+        return
+    fi
+
+    # Second, search the open reviews returned by 'arc list' looking
+    # for a subject match.
+    title=$(git show -s --format=%s $commit)
+    diff=$(arc list | fgrep "$title" | egrep -o 'D[1-9][0-9]*:' | tr -d ':')
+    if [ -z "$diff" ]; then
+        err "could not find review for '${title}'"
+    elif [ $(echo "$diff" | wc -l) -ne 1 ]; then
+        err "found multiple reviews with the same title"
+    fi
+
+    echo $diff
+}
+
+create_one_review()
+{
+    local childphid commit dir doprompt msg parent parentphid reviewers
+    local subscribers
+
+    commit=$1
+    reviewers=$2
+    subscribers=$3
+    parent=$4
+    doprompt=$5
+
+    if [ "$doprompt" ] && ! show_and_prompt $commit; then
+        return 1
+    fi
+
+    git checkout -q $commit
+
+    dir=$(git rev-parse --git-dir)/arc
+    mkdir -p "$dir"
+
+    msg=${dir}/create-message
+    git show -s --format='%B' $commit > $msg
+    printf "\nTest Plan:\n" >> $msg
+    printf "\nReviewers:\n" >> $msg
+    printf "${reviewers}\n" >> $msg
+    printf "\nSubscribers:\n" >> $msg
+    printf "${subscribers}\n" >> $msg
+
+    yes | env EDITOR=true \
+        arc diff --never-apply-patches --create --allow-untracked $BROWSE HEAD~
+    [ $? -eq 0 ] || err "could not create Phabricator diff"
+
+    if [ -n "$parent" ]; then
+        diff=$(commit2diff $commit)
+        [ -n "$diff" ] || err "failed to look up review ID for $commit"
+
+        childphid=$(diff2phid $diff)
+        parentphid=$(diff2phid $parent)
+        echo '{
+            "objectIdentifier": "'${childphid}'",
+            "transactions": [
+                {
+                    "type": "parents.add",
+                    "value": ["'${parentphid}'"]
+                }
+             ]}' |
+            arc call-conduit -- differential.revision.edit >&3
+    fi
+    return 0
+}
+
+# Get a list of reviewers who accepted the specified diff.
+diff2reviewers()
+{
+    local diff phid reviewid userids
+
+    diff=$1
+    reviewid=$(diff2phid $diff)
+    userids=$( \
+        echo '{
+                  "constraints": {"phids": ["'$reviewid'"]},
+                  "attachments": {"reviewers": true}
+              }' |
+        arc call-conduit -- differential.revision.search |
+        jq '.response.data[0].attachments.reviewers.reviewers[] | select(.status == "accepted").reviewerPHID')
+    if [ -n "$userids" ]; then
+        echo '{
+                  "constraints": {"phids": ['$(echo -n $userids | tr '[:space:]' ',')']}
+              }' |
+            arc call-conduit -- user.search |
+            jq -r '.response.data[].fields.username'
+    fi
+}
+
+prompt()
+{
+    local resp
+
+    if [ "$ASSUME_YES" ]; then
+        return 1
+    fi
+
+    printf "\nDoes this look OK? [y/N] "
+    read resp
+
+    case $resp in
+    [Yy])
+        return 0
+        ;;
+    *)
+        return 1
+        ;;
+    esac
+}
+
+show_and_prompt()
+{
+    local commit
+
+    commit=$1
+
+    git show $commit
+    prompt
+}
+
+save_head()
+{
+    local commit orig
+
+    if ! orig=$(git symbolic-ref --short -q HEAD); then
+        orig=$(git show -s --pretty=%H HEAD)
+    fi
+    echo $orig
+}
+
+restore_head()
+{
+    local orig
+
+    orig=$1
+    git checkout -q $orig
+}
+
+build_commit_list()
+{
+    local chash _commits commits
+
+    for chash in $@; do
+        _commits=$(git rev-parse "${chash}")
+        if ! git cat-file -e "${chash}"'^{commit}' >/dev/null 2>&1; then
+            _commits=$(git rev-list $_commits | tail -r)
+        fi
+        [ -n "$_commits" ] || err "invalid commit ID ${chash}"
+        commits="$commits $_commits"
+    done
+    echo $commits
+}
+
+gitarc::create()
+{
+    local commit commits doprompt list o orig prev reviewers
+    local subscribers
+
+    list=
+    if eval $(git config --bool --default false --get arc.list); then
+        list=1
+    fi
+    doprompt=1
+    while getopts lr:s: o; do
+        case "$o" in
+        l)
+            list=1
+            ;;
+        r)
+            reviewers="$OPTARG"
+            ;;
+        s)
+            subscribers="$OPTARG"
+            ;;
+        *)
+            err_usage
+            ;;
+        esac
+    done
+    shift $((OPTIND-1))
+
+    commits=$(build_commit_list $@)
+
+    if [ "$list" ]; then
+        for commit in ${commits}; do
+            git --no-pager show --oneline --no-patch $commit
+        done | git_pager
+        if ! prompt; then
+            return
+        fi
+        doprompt=
+    fi
+
+    orig=$(save_head)
+    prev=""
+    for commit in ${commits}; do
+        if create_one_review "$commit" "$reviewers" "$subscribers" "$prev" \
+                             "$doprompt"; then
+            prev=$(commit2diff "$commit")
+        else
+            prev=""
+        fi
+    done
+    restore_head $orig
+}
+
+gitarc::list()
+{
+    local chash commit commits diff title
+
+    commits=$(build_commit_list $@)
+
+    for commit in $commits; do
+        chash=$(git show -s --format='%C(auto)%h' $commit)
+        echo -n "${chash} "
+
+        diff=$(log2diff $commit)
+        if [ -n "$diff" ]; then
+                diff2status $diff
+                continue
+        fi
+
+        # This does not use commit2diff as it needs to handle errors
+        # differently and keep the entire status.  The extra 'cat'
+        # after 'fgrep' avoids erroring due to -e.
+        title=$(git show -s --format=%s $commit)
+        diff=$(arc list | fgrep "$title" | cat)
+        if [ -z "$diff" ]; then
+            echo "No Review      : $title"
+        elif [ $(echo "$diff" | wc -l) -ne 1 ]; then
+            echo -n "Ambiguous Reviews: "
+            echo "$diff" | egrep -o 'D[1-9][0-9]*:' | tr -d ':' \
+                | paste -sd ',' - | sed 's/,/, /g'
+        else
+            echo "$diff" | sed -e 's/^[^ ]* *//'
+        fi
+    done
+}
+
+gitarc::patch()
+{
+    local rev
+
+    if [ $# -eq 0 ]; then
+        err_usage
+    fi
+
+    for rev in $@; do
+        arc patch --skip-dependencies --nocommit --nobranch --force $rev
+        echo "Applying ${rev}..."
+        [ $? -eq 0 ] || break
+    done
+}
+
+gitarc::stage()
+{
+    local branch commit commits diff reviewers tmp
+
+    branch=main
+    while getopts b: o; do
+        case "$o" in
+        b)
+            branch="$OPTARG"
+            ;;
+        *)
+            err_usage
+            ;;
+        esac
+    done
+    shift $((OPTIND-1))
+
+    commits=$(build_commit_list $@)
+
+    if [ "$branch" = "main" ]; then
+        git checkout -q main
+    else
+        git checkout -q -b ${branch} main
+    fi
+
+    tmp=$(mktemp)
+    for commit in $commits; do
+        git show -s --format=%B $commit > $tmp
+        diff=$(arc list | fgrep "$(git show -s --format=%s $commit)" |
+            egrep -o 'D[1-9][0-9]*:' | tr -d ':')
+        if [ -n "$diff" ]; then
+            # XXX this leaves an extra newline in some cases.
+            reviewers=$(diff2reviewers $diff | sed '/^$/d' | paste -sd ',' - | sed 's/,/, /g')
+            if [ -n "$reviewers" ]; then
+                printf "Reviewed by:\t${reviewers}\n" >> $tmp
+            fi
+            printf "Differential Revision:\thttps://reviews.freebsd.org/${diff}" >> $tmp
+        fi
+        if ! git cherry-pick --no-commit ${commit}; then
+            warn "Failed to apply $(git rev-parse --short ${commit}).  Are you staging patches in the wrong order?"
+            git checkout -f
+            break
+        fi
+        git commit --edit --file $tmp
+    done
+}
+
+gitarc::update()
+{
+    local commit diff orig
+
+    commit=$1
+    diff=$(commit2diff $commit)
+
+    if ! show_and_prompt $commit; then
+        return
+    fi
+
+    orig=$(save_head)
+    git checkout -q $commit
+
+    # The linter is stupid and applies patches to the working copy.
+    # This would be tolerable if it didn't try to correct "misspelled" variable
+    # names.
+    arc diff --allow-untracked --never-apply-patches --update $diff HEAD~
+
+    restore_head $orig
+}
+
+set -e
+
+ASSUME_YES=
+if eval $(git config --bool --default false --get arc.assume-yes); then
+    ASSUME_YES=1
+fi
+
+VERBOSE=
+while getopts vy o; do
+    case "$o" in
+    v)
+        VERBOSE=1
+        ;;
+    y)
+        ASSUME_YES=1
+        ;;
+    *)
+        err_usage
+        ;;
+    esac
+done
+shift $((OPTIND-1))
+
+[ $# -ge 1 ] || err_usage
+
+which arc >/dev/null 2>&1 || err "arc is required, install devel/arcanist"
+which jq >/dev/null 2>&1 || err "jq is required, install textproc/jq"
+
+if [ "$VERBOSE" ]; then
+    exec 3>&1
+else
+    exec 3> /dev/null
+fi
+
+case "$1" in
+create|list|patch|stage|update)
+    ;;
+*)
+    err_usage
+    ;;
+esac
+verb=$1
+shift
+
+# All subcommands require at least one parameter.
+if [ $# -eq 0 ]; then
+    err_usage
+fi
+
+# Pull in some git helper functions.
+git_sh_setup=$(git --exec-path)/git-sh-setup
+[ -f "$git_sh_setup" ] || err "cannot find git-sh-setup"
+SUBDIRECTORY_OK=y
+USAGE=
+. "$git_sh_setup"
+
+# Bail if the working tree is unclean, except for "list" and "patch"
+# operations.
+case $verb in
+list|patch)
+    ;;
+*)
+    require_clean_work_tree $verb
+    ;;
+esac
+
+if eval $(git config --bool --default false --get arc.browse); then
+    BROWSE=--browse
+fi
+
+gitarc::${verb} $@