✎ soc:2008:stefanha:journal:start

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====== Stefan Hajnoczi: GDB Remote Debugging ======

===== Journal =====

==== Week 1 ====
**Milestone:** Set up IDT and write an interrupt handler.

=== Fri May 23 ===
Some notes after chatting with mdc and mcb30:
  * Place IDT code in ''arch/i386/transitions/librm.S'' - in similar places to ''lgdt'' and ''sgdt''.
  * GDB stub should be written with portability in mind, separate out arch-specific parts.
  * GDB stub should be a build option.
  * Source-level debugging and symbols should work (mostly) out-of-the-box due to ELF build.

=== Sat May 24 ===
Git commit: [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=ac29ad53aff6e89f12bd5a163861d1afb1846049|ac29ad53aff6e89f12bd5a163861d1afb1846049]]

Implemented an interrupt handler in ''arch/i386/transitions/librm.S''.  It currently sets ''eax'' to ''0xcafebabe'' and spins in an infinite loop.  Have tested that it is working using QEMU.

=== Sun May 25 ===
Git commits: [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=12144ffbfadef9c6597f9ac754685223bb736368|12144ffbfadef9c6597f9ac754685223bb736368]], [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=1ff72edaa0c68966e1bc102ae5167d714eeb03e6|1ff72edaa0c68966e1bc102ae5167d714eeb03e6]], [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=c15542a614961acc1051296fc2367d1539db57ff|c15542a614961acc1051296fc2367d1539db57ff]]

When GDB reads or writes registers on x86, it wants a snapshot like this: ''EAX'', ''ECX'', ''EDX'', ''EBX'', ''ESP'', ''EBP'', ''ESI'', ''EDI'', ''EIP'', ''EFLAGS'', ''CS'', ''SS'', ''DS'', ''ES'', ''FS'', ''GS''.  This snapshot is a blob that gets sent between the GDB stub and GDB.

The interrupt handler now takes this register snapshot and passes it to the GDB stub.  It also applies the register snapshot to the actual CPU state when the GDB stub returns.  So if the GDB stub changes ''EAX'' in the register snapshot, then the ''EAX'' register will be changed when the interrupt handler returns.

Each interrupt is mapped onto a POSIX signal number (e.g. ''SIGSEGV'').  The GDB protocol communicates these numbers when reporting that execution was interrupted.

=== Mon May 26 ===
Git commit: [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=f6c6b14468fffff0cf55df77ee7bb796113bcb4a|f6c6b14468fffff0cf55df77ee7bb796113bcb4a]], [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=6f8c3b03af1fa4733958a0ad66496a0acc8ce882|6f8c3b03af1fa4733958a0ad66496a0acc8ce882]]

Asked mcb30 for feedback on the code so far.  The latest git commit includes his suggested clean ups and simplifications.

The interrupt handler calls ''gdbstub_handler(regs)'', where ''regs'' is a pointer to the register snapshot.  The GDB stub may change the values in the register snapshot.  When the interrupt handler exits, it applies the snapshot to the CPU state.  **Changing ''ESP'' is currently not supported**, since it is more difficult to implement and we do not anticipate it ever being changed.

=== Tue May 27 ===
Git commit: [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=d1e823a19d9c847fb7a965f8fbb9345f68875c3a|d1e823a19d9c847fb7a965f8fbb9345f68875c3a]]

The GDB stub has initial support for:
  * Register read/write
  * Memory read/write
  * Continue and step
  * Breakpoints
  * Source-level debugging

Here is a screenshot:

{{:soc:2008:stefanha:journal:gdbstub.png|Early version of the GDB stub in action.}}

The stub currently uses the serial driver directly.  I need to design a clean GDB transport interface.

To try it out:
<code>
$ git clone git://git.etherboot.org/scm/people/stefanha/gpxe.git gdbstub
$ cd gdbstub/src
$ make
$ qemu -serial tcp::4444,server bin/gpxe.usb

[From a different terminal]
$ cd gdbstub/src
$ gdb
(gdb) file bin/gpxe.hd.tmp
(gdb) target remote localhost:4444
</code>

=== Wed May 28 ===
Git commit: [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=6f5d000a673209278400b9a04e12ee36cab07d28|6f5d000a673209278400b9a04e12ee36cab07d28]]

Slow day today because I need to do university work for Thurs and Fri, my last ever assessment.  Will get back to gPXE tomorrow afternoon.

Talked to mcb30 about improvements to the GDB stub.  Checked that the register ordering is indeed correct and that GDB uses the reverse order to the pushal instruction.

I tried running a process that polls for serial activity and breaks into the GDB stub on activity.  This eliminates the need for a hardcoded breakpoint during gPXE startup.  gPXE will boot normally when GDB is not being used.  If GDB is connected, then it will break into the GDB stub.

I am not sure if this is the best solution, but I'll use it for a while and see how effective it is.

=== Thur May 29 ===
Git commits: [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=d828b65e7182372fdf3f9174f76a6d31341d16a9|d828b65e7182372fdf3f9174f76a6d31341d16a9]], [[http://git.etherboot.org/?p=people/stefanha/gpxe.git;a=commit;h=00f6bbbb61348865d57ebc979617f5f962bd037c|00f6bbbb61348865d57ebc979617f5f962bd037c]]

I cleaned up the code today and did manual testing to see if things behave the way they are expected to.

Moved register snapshot and single step support into its own header file ''arch/i386/include/gdbmach.h''.  This keeps the GDB stub portable.

Implemented retransmit support if GDB sends NACK after receiving a corrupted reply from the GDB stub.

=== Fri May 30 ===

Had the weekly mentor meeting today with mdc.  The aim for next week is **to push the GDB stub into mainline**.  Mainline support will make using the GDB stub easy for developers and advanced users.  I hope that others will find debugging useful way to develop faster.  Mainline exposure will also help the GDB stub to improve.

The mainline effort requires making the GDB stub a ''config.h'' option and writing usage documentation.  I think the GDB stub should not be enabled in default builds since only a small number of users will ever need it.  When these tasks are complete I will ask mcb30 to consider merging it into mainline.

I have been reading the [[http://sourceware.org/gdb/current/onlinedocs/gdb_toc.html|GDB Manual]] and playing with GDB command files (aka scripts).  I am writing a test suite for the GDB stub using GDB scripts.

To run the test suite you start gPXE and launch GDB with the test suite scripts.  The scripts step through a series of test procedures, like writing to memory and reading that memory back to check the write was correctly performed.

Having a test suite is important to me because I want to have the freedom of experimenting with the GDB stub code without worrying about breaking things.  The test suite will make sure I don't introduce regressions.

Next steps:
  * Write a test suite.
  * Isolate the interrupt handling code from the rest of ''librm.S'' to avoid code size penalty in non-GDB builds.
  * Add config option for GDB stub.
  * Make ''localconfig.h'' that gets sourced from ''config.h'' and ignored by git.  This will prevent people mistakingly checking in debug options into git.
  * Design a GDB protocol transport interface that serial and UDP can implement.  Discuss with mentors.
  * Memory read/write should probably support device memory.  Do devices require 32-bit/16-bit reads to give sane results?
  * Handle disconnect properly - ignore breakpoints when GDB is not attached.
Trace: • nikhilcrao • hook_bios_interrupt • journal • week4 • journal • week12 • indolent_playground • under_construction
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