Re: [Qemu-devel] Can't switch to 64bit mode at boot, how to debug?

[Max Filippov]

On Sun, Nov 18, 2012 at 2:09 AM, Goswin von Brederlow <address@hidden> wrote:
> On Sat, Nov 17, 2012 at 10:58:17PM +0300, Max Filippov wrote:
>> On Sat, Nov 17, 2012 at 9:50 PM, Goswin von Brederlow <address@hidden> wrote:

[...]

>> (gdb) b *0x100000
>> Breakpoint 1 at 0x100000
>> (gdb) c
>> Continuing.
>
> That works, but only for addresses BEFORE where it crashes. What works, but
> in no way helps (see below), it setting the breakpoint before the last
> instruction and then "stepi".
>
> Only way that would help would be entering the monitor mode on such an error.

Ok, I think I can cook a patch for that, but how are you going to use it?
The issue is not reproducible with mainline qemu, neither current nor v1.1.0

[...]

>> Looks like PAE support is broken.
>
> Yeah, any memory read just returns all zeroes. That threw me too. I had
> to debug this by adding "mov $0x101000,%ebx" type instructions and then
> check the contents of ebx on crash. I wagely remember this always being
> broken.
>
> My assumption would be that I'm setting up the gdt, paging, the page tables
> and such and the debug mode already tries to use them while the cpu is
> still in the original mode. So the two don't agree on what is where.

There's a bug related to PAE in the debug interface, I've posted a patch that
should fix that:
https://lists.gnu.org/archive/html/qemu-devel/2012-11/msg01635.html

-- 
Thanks.
-- Max