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[Qemu-devel] [Bug 640213] Re: QEMU does not communicate properly with GD

From: Thomas Huth
Subject: [Qemu-devel] [Bug 640213] Re: QEMU does not communicate properly with GDB with a 64 bit guest
Date: Wed, 18 Jan 2017 15:58:31 -0000

Can you still reproduce this problem with the latest version of QEMU, or
can we close this ticket nowadays?

** Changed in: qemu
       Status: New => Incomplete

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  QEMU does not communicate properly with GDB with a 64 bit guest

Status in QEMU:

Bug description:
  I have been trying to figure out why I cannot debug a 64 bit kernel of
  my own invention.

  I launch qemu-system-x86_64 with the -s -S flags, we also specify -cpu
  core2duo -vga std and a -hda with an ext2 FS holding our multiboot
  kernel and GRUB2.

  When I try to set breakpoints and "continue" in GDB (7.2) using the
  very latest HEAD (b6601141cd2a170dfe773987b06f716a190ea7e0) or 0.12.0
  or 0.12.5 or 13.0.rc0 or 13.0.rc1, I get failures of the same nature:

  0x0000000000000000 in ?? ()
  (gdb) break main
  Breakpoint 1 at 0x101730: file src/kernel/init.c, line 18.
  (gdb) c

  Program received signal SIGTRAP, Trace/breakpoint trap.
  0x0000000000000000 in ?? ()

  Note that in this case, main lies in 64 bit mode. However, trying to
  break on _start yields virtually the same effect and _start is 32 bit

  By doing a git bisect, I managed to narrow the commit that introduced this 
bug to 5f30fa18ad043a841fe9f0c3917ac60f2519ebd1. Reverting this commit on HEAD 
seemingly fixed the problem for both the 32 bit and 64 bit cases.
  I might be doing something incorrectly on my end but this seemed to fix the 

  Perhaps the pertinent thing to do would be to revert
  5f30fa18ad043a841fe9f0c3917ac60f2519ebd1 as it seems to do nothing but
  break things unless, of course, this would only break something that I
  am not aware of further.

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