Debugging a 64-bit kernel in qemu

Jan Kiszka jan.kiszka@web.de
Mon Jan 3 13:00:00 GMT 2011


[ please keep CCs ]

Am 03.01.2011 13:27, Markus Duft wrote:
> On 01/03/2011 01:15 PM, Markus Duft wrote:
>> On 01/03/2011 12:15 PM, Jan Kiszka wrote:
>> [snip]
> [snip]
>> actually, i find that Ted Harkington was right: in 0.11.1 i can debug 32 bit code with qemu-system-x86_64 well enough (which means i debugged all the 32 bit part of my kernel without ever seen _any_ problem/non-working feature/whatever). wouldn't it be better to have 64 bit debugging working in the 64 bit version, with 32 bit mode working mostly (with whatever small issues), rather than just completely dooming 64 bit debugging...?
>>
> 
> owh - spoke too soon. there must be more to it: i tried reverting 5f30fa18ad043a841fe9f0c3917ac60f2519ebd1, which restores ability to debug my 64 bit kernel just fine, but now i get the packet too long when trying to debug 32 bit code....

Hmm, that's new. You definitely loose stack unwinding when using the
wrong mode, thus source-level debugging.

I thought that thread suggested to set the arch explicitly, maybe I
misremembered that:

set arch i386:x86_64
tar rem :1234

If that is required, you probably load a 32-bit binary into gdb that
also contains 64-bit code in some section. I guess this is even more
confusing for gdb.

> 
> wouldn't it be possible to implement some kind of explicit switch with qemu in the meantime, so i can choose what bitness i want to debug? I know, it's a problem with gdb under the hood, but still - it's really uncool debugging doesn't work in either of the two cases.

Wasn't required so far. If you debug in either mode, "set arch" should
do the job. If you have to debug across mode switches, that knob won't
help anyway.

Jan

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