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Re: Debugging a 64-bit kernel in qemu

Am 03.01.2011 11:27, Markus Duft wrote:
> Hi!
> I have been playing a little with this: I'm writing a kernel for both x86 and x86-64. While doing so, i'd like to debug the kernel using qemu (and it's gdb stub) and gdb. This worked very well until qemu-0.11.1 (gdb version does not seem to play any role...). From there on, debugging the 64 bit version no longer works. My sessions look like this with qemu-0.13.50 (and any version above 0.11.1, actually...):
> mduft@s01en22 /big/Privat/osdev/tachyon2 $ x86_64-pc-linux-gnu-gdb .build/x86_64/x86_64-tachyon
> GNU gdb (GDB)
> [snip]
> Reading symbols from /big/Privat/osdev/tachyon2/.build/x86_64/x86_64-tachyon...done.
> (gdb) target remote:1234
> Remote debugging using :1234
> 0x0000000000000000 in ?? ()
> (gdb) b boot
> Breakpoint 1 at 0xffffffff80119000: file /big/Privat/osdev/tachyon2/core/tachyon.boot/, line 25.
> (gdb) c
> Continuing.
> Remote 'g' packet reply is too long: 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
>  0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> 000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000801f0000
> (gdb) quit
> actuallly, i _can_ debug the kernel, nut only until the long mode switch occurs. after this point: no luck... :[
> now for the questions:
> 1) is this a problem with qemu or was qemu "fixed" and gdb has a problem?
>    (that's why i CCd the gdb list ;)).
> 2) is there any plan to fix this issue?
> 3) is there some kind of workaround i can use (i'd be happy with an ugly/unsupported patch too, since i build all my stuff myself ;)).

You see the effects of a workaround in qemu for gdb shortcomings on x86.
See e.g., there
should be some threads on the gdb list as well.


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