Re: [Qemu-devel] Failed to set a breakpoint on start_kernel

[Jan Kiszka]

[ 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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