on ubuntu:15.10, installed hhvm 1.13.1
updated /etc/hhvm/php.ini with the following:

; php options
;
session.save_handler = files
session.save_path = /var/lib/php5
session.gc_maxlifetime = 1440

; hhvm specific
;hhvm.jit_warmup_requests = 10

hhvm.server.port=9000
hhvm.server.type=fastcgi
hhvm.server.source_root=/var/www/leonardo/public
hhvm.server.default_document = index.php

hhvm.log.use_log_file = true
hhvm.log.level = Verbose
hhvm.log.file = /var/log/hhvm/error.log
hhvm.log.header = true
hhvm.log.use_sys_log = true
hhvm.log.injected_stack_trace = true
hhvm.log.native_stack_trace = true
hhvm.log.always_log_unhandled_exceptions = true
hhvm.log.runtime_error_reporting_level = 1
hhvm.mysql.typed_results = false

hhvm.hack.lang.look_for_typechecker = false
hhvm.utf8 = true

xdebug.dump.SERVER = *

; Xdebug options
xdebug.enable=1
xdebug.remote_enable=1
xdebug.idekey="xyz"
xdebug.remote_host="localhost"
xdebug.remote_handler=dbgp
xdebug.remote_port=9001
xdebug.remote_autostart=1
xdebug.remote_connect_back=1

xdebug.profiler_enable=0
xdebug.profiler_append=0

xdebug.collect_includes=0
xdebug.profiler_output_name=cachegrind.out.%p.%s
xdebug.profiler_output_dir=/var/www/leonardo/storage/cachegrind

xdebug.auto_trace=1
#xdebug.collect_params=3
#xdebug.show_mem_delta=1
xdebug.trace_output_dir=/var/www/leonardo/storage/cachegrind
xdebug.trace_output_name="trace.%t.%R"
xdebug.extended_info=1
xdebug.trace_format=1

Run a web application from a browser
Check the trace_output_dir for updated trace files with request uri