Re: [Qemu-devel] [Xen-devel] Project idea: make QEMU more flexible

[Paolo Bonzini <pbonzini@redhat.com>]

Il 23/01/2014 17:23, Wei Liu ha scritto:
> On Thu, Jan 23, 2014 at 01:54:40PM +0000, Wei Liu wrote:
>> On Thu, Jan 23, 2014 at 10:11:57AM +0100, Paolo Bonzini wrote:
>>> Il 22/01/2014 17:09, Wei Liu ha scritto:
>>>> On Wed, Jan 22, 2014 at 11:20:38AM +0100, Paolo Bonzini wrote:
>>>>> Il 21/01/2014 19:27, Wei Liu ha scritto:
>>>>>>>>
>>>>>>>> Googling "disable tcg" would have provided an answer, but the patches
>>>>>>>> were old enough to be basically useless.  I'll refresh the current
>>>>>>>> version in the next few days.  Currently I am (or try to be) on
>>>>>>>> vacation, so I cannot really say when, but I'll do my best. :)
>>>>>>>>
>>>>>> Hi Paolo, any update?
>>>>>
>>>>> Oops, sorry, I thought I had sent that out.  It's in the disable-tcg
>>>>> branch on my github repository.
>>>>>
>>>>
>>>> Unfortunately your branch didn't work when I enabled TCG support. If I
>>>> use "--disable-tcg" with configure then it works fine.
>>>
>>> Branch fixed.
>>>
>>
>> Yes, it's fixed for the case I reported. Thanks.
>>
>> But it is now broken with following rune:
>> ./configure --enable-kvm --disable-tcg --target-list=i386-softmmu
>> --disable-xen --enable-debug
>>
>>   LINK  i386-softmmu/qemu-system-i386
>>   cpus.o: In function `cpu_signal':
>>   /local/scratch/qemu/cpus.c:569: undefined reference to `exit_request'
>>   cpus.o: In function `tcg_cpu_exec':
>>   /local/scratch/qemu/cpus.c:1257: undefined reference to `cpu_x86_exec'
>>   cpus.o: In function `tcg_exec_all':
>>   /local/scratch/qemu/cpus.c:1282: undefined reference to `exit_request'
>>   /local/scratch/qemu/cpus.c:1299: undefined reference to `exit_request'
>>   exec.o: In function `tlb_reset_dirty_range_all':
>>   /local/scratch/qemu/exec.c:736: undefined reference to
>>   `cpu_tlb_reset_dirty_all'
>>   collect2: error: ld returned 1 exit status
>>   make[1]: *** [qemu-system-i386] Error 1
>>   make: *** [subdir-i386-softmmu] Error 2
>>
>> --enable-debug is the one to blame. Without that it links successfully.
>>
>> Wei.
>>
>
> Finally I figured out what was wrong. Your patch series was relying on
> compiler to aggresively optimize away unused code.
>
> So when --enable-debug is set, compiler won't optimize away the dead
> code, hence those undefine references. With any optimization option -O
> you series compiles successfully.
>
> Feel free to integrate my patch below, or fix those errors in the way
> you see appropriate.

Thanks!  I added stubs for all three undefined symbols in tcg-stub.c.

Another way to fix it would be -ffunction-sections/-Wl,--gc-sections 
(which shaves 200k more out of the .text section), but that breaks glibc 
static linking.

Paolo