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

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

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

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