Re: [Qemu-devel] emulation details of qemu

[Alex Bennée]

tutu sky <address@hidden> writes:

> Thank you in advance Alex.
> you said: "Using the QEMU's gdbstub to debug a guest is different from 
> debugging QEMU by running it under gdb."
> if i want to see the hardware's internal which is emulated by QEMU,

If by hardware's internal state then yes, GDB doesn't export much other
than the system registers. However you can extend the gdbstub to make
more machine state visible to gdb. See the gdm-xml directory for
examples of defining additional registers for GDB and the various
target-${foo}/gdbstub.c files which pas those register values back and
forth.


>i must make QEMU to run in step mode and run QEMU under GDB, no matter which 
>guest is running; but if i want to debug a gust, QEMU makes it easy for me by 
>offering "gdbstub" and i may need to compile the kernel from source, Do i 
>understand you right?
>
> ________________________________________
> From: Alex Bennée <address@hidden>
> Sent: Friday, April 29, 2016 3:08 PM
> To: tutu sky
> Cc: Stefan Hajnoczi; address@hidden
> Subject: Re: [Qemu-devel] emulation details of qemu
>
> tutu sky <address@hidden> writes:
>
>> Magic answer, Thanks a lot Alex.
>> you mean GDB will be enabled for just QEMU's itself internals? It does not 
>> make importance or any difference for guest running on it?
>> if i want describe my opinion in another way, i think you said that
>> when enabling GDB for QEMU, it is usable and is just important to be
>> usable for QEMU internals, as a user wants to develop it or a person
>> may want to know how he can watch a processor internals. Yeah?
>
> I'm not sure I follow. Using the QEMU's gdbstub to debug a guest is
> different from debugging QEMU by running it under gdb.
>
>> Can GDB  be activated for multicore architectures? in order to see every 
>> core's internals separately?
>> I ask these questions because QEMU documentation is not clear enough
>> and sometimes hard to understand. for example for attaching GDB to
>> QEMU, i am unable to find a good and general guide. it seems it just
>> depend on how much you know about GDB and how to work with. am i
>> right?
>
> Generally to use the stub you start the guest with -s -S, e.g:
>
> qemu-system-arm -machine virt,accel=tcg -cpu cortex-a15 -display none \
>   -serial stdio -kernel ./arm/locking-test.flat -smp 4 -s -S
>
> And then invoke gdb with something like:
>
> gdb-multiarch ./arm/locking-test.elf -ex "target remote localhost:1234"
>
> So in this example I'm using the .elf file with gdb as that has the
> debugging information for the .flat file I started QEMU with. -ex just
> saves the hassle of typing in the "target remote localhost:1234" to
> connect to the gdb stub when you start up. Once in gdb you can do all
> the usual things:
>
> (gdb) info threads
>   Id   Target Id         Frame
>   * 1    Thread 1 (CPU#0 [running]) 0x40000000 in ?? ()
>     2    Thread 2 (CPU#1 [halted ]) 0x00000000 in ?? ()
>     3    Thread 3 (CPU#2 [halted ]) 0x00000000 in ?? ()
>     4    Thread 4 (CPU#3 [halted ]) 0x00000000 in ?? ()
> (gdb) x/4i $pc
>   => 0x40000000:  mov     r0, #0
>      0x40000004:  ldr     r1, [pc, #4]    ; 0x40000010
>      0x40000008:  ldr     r2, [pc, #4]    ; 0x40000014
>      0x4000000c:  ldr     pc, [pc, #4]    ; 0x40000018
> (gdb) p/x $r0
> $1 = 0x0
> (gdb) p/x $r1
> $2 = 0x0
> (gdb) i
>      0x40000004 in ?? ()
>   => 0x40000004:  ldr     r1, [pc, #4]    ; 0x40000010
>      0x40000008:  ldr     r2, [pc, #4]    ; 0x40000014
>      0x4000000c:  ldr     pc, [pc, #4]    ; 0x40000018
> (gdb) i
>      0x40000008 in ?? ()
>   => 0x40000008:  ldr     r2, [pc, #4]    ;
>      0x40000014
> (gdb) p/x $r1
> $3 = 0xffffffff
>
>>
>> Thanks and regards.
>>
>> ________________________________________
>> From: Alex Bennée <address@hidden>
>> Sent: Friday, April 29, 2016 12:22 PM
>> To: tutu sky
>> Cc: Stefan Hajnoczi; address@hidden
>> Subject: Re: [Qemu-devel] emulation details of qemu
>>
>> tutu sky <address@hidden> writes:
>>
>>> Yeah, thank you Alex.
>>> If I use a linux on top of the qemu, for entering debug mode, do i
>>> need to compile kernel from source or it is not dependent on debugging
>>> qemu itself?
>>
>> I'm not sure I follow. As far as QEMU is concerned it provides a stub
>> for GDB to talk to and doesn't need to know anything else about the
>> guest it is running. The GDB itself will want symbols one way or another
>> so you would either compile your kernel from source or pass the debug
>> symbol enabled vmlinux to GDB using symbol-file.
>>
>>> and then is it possible to define a heterogeneous multicore platform
>>> in qemu?
>>
>> The current upstream QEMU doesn't support heterogeneous setups although
>> some preliminary work has been posted to allow multiple front-ends to be
>> compiled together.
>>
>> There are certainly out-of-tree solutions although as I understand it
>> (I've not worked with them myself) they use multiple QEMU runtimes
>> linked together with some sort of shared memory bus/IPC layer.
>>
>>>
>>> Thanks and regards.
>>>
>>> ________________________________________
>>> From: Alex Bennée <address@hidden>
>>> Sent: Thursday, April 28, 2016 6:45 PM
>>> To: tutu sky
>>> Cc: Stefan Hajnoczi; address@hidden
>>> Subject: Re: [Qemu-devel] emulation details of qemu
>>>
>>> tutu sky <address@hidden> writes:
>>>
>>>> Thanks a lot Stefan,
>>>> But if i want to change the content of a register during run time in
>>>> debug mode, what should i do? is it possible at first?
>>>
>>> Using the gdbstub sure you can change the register values when the
>>> machine is halted.
>>>
>>>>
>>>> Regards.
>>>> ________________________________________
>>>> From: Stefan Hajnoczi <address@hidden>
>>>> Sent: Tuesday, April 26, 2016 9:31 AM
>>>> To: tutu sky
>>>> Cc: address@hidden
>>>> Subject: Re: [Qemu-devel] emulation details of qemu
>>>>
>>>> On Sat, Apr 23, 2016 at 06:36:39AM +0000, tutu sky wrote:
>>>>> I want to know that is it possible to access registers or 
>>>>> micro-architectural part of a core/cpu in qemu during run time?
>>>>
>>>> Yes.  How and to what extent depends on whether you are using TCG, KVM,
>>>> or TCI.  QEMU also has gdbstub support so you can single-step execution
>>>> and access CPU registers.
>>>>
>>>> Stefan


--
Alex Bennée