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Re: [Qemu-devel] Failed to set a breakpoint on start_kernel
From: |
Jan Kiszka |
Subject: |
Re: [Qemu-devel] Failed to set a breakpoint on start_kernel |
Date: |
Sat, 17 Mar 2012 14:31:29 +0100 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
[ re-added qemu-devel to CC ]
On 2012-03-17 13:10, Wei Yang wrote:
>> Two major issues with this procedure:
>>
>> 1. When using kvm, a soft breakpoint (as set by 'b') will inject a trap
>> instruction into the guest image - which is not yet loaded after the
>> bios ran. You need to use a hardware breakpoint in this case.
>>
>> 2. Due to gdb limitations, you cannot switch between 16/32-bit mode (the
>> CPU starts in 16 bit) and the 64-bit mode of kernel within the same gdb
>> session. Therefore:
>> - let the target run into Linux is active
>> - attach gdb
>> - issue "hw start_kernel"
>> - reboot (e.g. "monitor system_reset")
>> - you will hit the breakpoint, and gdb will be usable
>>
>> Jan
>>
>>
> oh, so when qemu run with kvm enabled, I couldn't debug the kernel right?
That's not what I said. You need to be aware of how it works. And, in
contrast to pure emulation, kwm uses a non-transparent mechanism for
injecting software breakpoints. Consider it the price for the gained speed.
>
> I tried to run qemu with out -enable-kvm, kernel could stop at the break
> point.
>
> BTW, I tried "hw start_kernel", but it failed.
> (gdb) hw start_kernel
> Undefined command: "hw". Try "help".
Sorry, typo. Must be "hb".
Jan
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