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Impact: prevents confusing the user when buffer size is inadequate The tracing framework offers a resizeable buffer, which mmiotrace uses to record events. If the buffer is full, the following events will be lost. Events should not be lost, so the documentation instructs the user to increase the buffer size. The buffer size is set via a debugfs file. Mmiotrace documentation was not updated the same time the debugfs file was changed. The old file was tracing/trace_entries and first contained the number of entries the buffer had space for, per cpu. Nowadays this file is replaced with the file tracing/buffer_size_kb, which tells the amount of memory reserved for the buffer, per cpu, in kilobytes. Previously, a flag had to be toggled via the debugfs file tracing/tracing_enabled when the buffer size was changed. This is no longer necessary. The mmiotrace documentation is updated to reflect the current state of the tracing framework. Signed-off-by: Pekka Paalanen <pq@iki.fi> Signed-off-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
164 lines
6.5 KiB
Plaintext
164 lines
6.5 KiB
Plaintext
In-kernel memory-mapped I/O tracing
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Home page and links to optional user space tools:
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http://nouveau.freedesktop.org/wiki/MmioTrace
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MMIO tracing was originally developed by Intel around 2003 for their Fault
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Injection Test Harness. In Dec 2006 - Jan 2007, using the code from Intel,
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Jeff Muizelaar created a tool for tracing MMIO accesses with the Nouveau
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project in mind. Since then many people have contributed.
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Mmiotrace was built for reverse engineering any memory-mapped IO device with
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the Nouveau project as the first real user. Only x86 and x86_64 architectures
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are supported.
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Out-of-tree mmiotrace was originally modified for mainline inclusion and
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ftrace framework by Pekka Paalanen <pq@iki.fi>.
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Preparation
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-----------
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Mmiotrace feature is compiled in by the CONFIG_MMIOTRACE option. Tracing is
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disabled by default, so it is safe to have this set to yes. SMP systems are
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supported, but tracing is unreliable and may miss events if more than one CPU
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is on-line, therefore mmiotrace takes all but one CPU off-line during run-time
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activation. You can re-enable CPUs by hand, but you have been warned, there
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is no way to automatically detect if you are losing events due to CPUs racing.
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Usage Quick Reference
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---------------------
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$ mount -t debugfs debugfs /debug
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$ echo mmiotrace > /debug/tracing/current_tracer
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$ cat /debug/tracing/trace_pipe > mydump.txt &
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Start X or whatever.
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$ echo "X is up" > /debug/tracing/trace_marker
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$ echo nop > /debug/tracing/current_tracer
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Check for lost events.
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Usage
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-----
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Make sure debugfs is mounted to /debug. If not, (requires root privileges)
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$ mount -t debugfs debugfs /debug
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Check that the driver you are about to trace is not loaded.
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Activate mmiotrace (requires root privileges):
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$ echo mmiotrace > /debug/tracing/current_tracer
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Start storing the trace:
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$ cat /debug/tracing/trace_pipe > mydump.txt &
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The 'cat' process should stay running (sleeping) in the background.
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Load the driver you want to trace and use it. Mmiotrace will only catch MMIO
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accesses to areas that are ioremapped while mmiotrace is active.
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During tracing you can place comments (markers) into the trace by
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$ echo "X is up" > /debug/tracing/trace_marker
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This makes it easier to see which part of the (huge) trace corresponds to
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which action. It is recommended to place descriptive markers about what you
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do.
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Shut down mmiotrace (requires root privileges):
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$ echo nop > /debug/tracing/current_tracer
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The 'cat' process exits. If it does not, kill it by issuing 'fg' command and
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pressing ctrl+c.
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Check that mmiotrace did not lose events due to a buffer filling up. Either
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$ grep -i lost mydump.txt
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which tells you exactly how many events were lost, or use
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$ dmesg
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to view your kernel log and look for "mmiotrace has lost events" warning. If
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events were lost, the trace is incomplete. You should enlarge the buffers and
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try again. Buffers are enlarged by first seeing how large the current buffers
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are:
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$ cat /debug/tracing/buffer_size_kb
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gives you a number. Approximately double this number and write it back, for
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instance:
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$ echo 128000 > /debug/tracing/buffer_size_kb
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Then start again from the top.
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If you are doing a trace for a driver project, e.g. Nouveau, you should also
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do the following before sending your results:
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$ lspci -vvv > lspci.txt
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$ dmesg > dmesg.txt
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$ tar zcf pciid-nick-mmiotrace.tar.gz mydump.txt lspci.txt dmesg.txt
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and then send the .tar.gz file. The trace compresses considerably. Replace
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"pciid" and "nick" with the PCI ID or model name of your piece of hardware
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under investigation and your nick name.
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How Mmiotrace Works
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-------------------
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Access to hardware IO-memory is gained by mapping addresses from PCI bus by
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calling one of the ioremap_*() functions. Mmiotrace is hooked into the
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__ioremap() function and gets called whenever a mapping is created. Mapping is
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an event that is recorded into the trace log. Note, that ISA range mappings
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are not caught, since the mapping always exists and is returned directly.
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MMIO accesses are recorded via page faults. Just before __ioremap() returns,
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the mapped pages are marked as not present. Any access to the pages causes a
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fault. The page fault handler calls mmiotrace to handle the fault. Mmiotrace
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marks the page present, sets TF flag to achieve single stepping and exits the
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fault handler. The instruction that faulted is executed and debug trap is
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entered. Here mmiotrace again marks the page as not present. The instruction
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is decoded to get the type of operation (read/write), data width and the value
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read or written. These are stored to the trace log.
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Setting the page present in the page fault handler has a race condition on SMP
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machines. During the single stepping other CPUs may run freely on that page
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and events can be missed without a notice. Re-enabling other CPUs during
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tracing is discouraged.
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Trace Log Format
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----------------
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The raw log is text and easily filtered with e.g. grep and awk. One record is
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one line in the log. A record starts with a keyword, followed by keyword
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dependant arguments. Arguments are separated by a space, or continue until the
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end of line. The format for version 20070824 is as follows:
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Explanation Keyword Space separated arguments
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---------------------------------------------------------------------------
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read event R width, timestamp, map id, physical, value, PC, PID
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write event W width, timestamp, map id, physical, value, PC, PID
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ioremap event MAP timestamp, map id, physical, virtual, length, PC, PID
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iounmap event UNMAP timestamp, map id, PC, PID
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marker MARK timestamp, text
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version VERSION the string "20070824"
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info for reader LSPCI one line from lspci -v
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PCI address map PCIDEV space separated /proc/bus/pci/devices data
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unk. opcode UNKNOWN timestamp, map id, physical, data, PC, PID
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Timestamp is in seconds with decimals. Physical is a PCI bus address, virtual
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is a kernel virtual address. Width is the data width in bytes and value is the
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data value. Map id is an arbitrary id number identifying the mapping that was
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used in an operation. PC is the program counter and PID is process id. PC is
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zero if it is not recorded. PID is always zero as tracing MMIO accesses
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originating in user space memory is not yet supported.
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For instance, the following awk filter will pass all 32-bit writes that target
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physical addresses in the range [0xfb73ce40, 0xfb800000[
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$ awk '/W 4 / { adr=strtonum($5); if (adr >= 0xfb73ce40 &&
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adr < 0xfb800000) print; }'
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Tools for Developers
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--------------------
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The user space tools include utilities for:
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- replacing numeric addresses and values with hardware register names
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- replaying MMIO logs, i.e., re-executing the recorded writes
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