HHVM with PHP-FPM fallback

hhvm.conf

location ~ \.(hh|php)$ {
    proxy_intercept_errors on;
    error_page 502 = @fallback;

    try_files $uri =404;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    if (!-f $document_root$fastcgi_script_name) {
        return 404;
    }

    fastcgi_keep_conn on;

    include fastcgi_params;

    fastcgi_index index.php;
    fastcgi_pass unix:/var/run/hhvm/server.sock;
}
location @fallback {
    try_files $uri =404;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    if (!-f $document_root$fastcgi_script_name) {
        return 404;
    }

    include fastcgi_params;

    fastcgi_index index.php;
    fastcgi_pass unix:/var/run/php5-fpm.sock;
}