Re: Illegal instruction reported by qemu-ppc with -cpu 7400

[Khem Raj]

On Thu, Mar 17, 2022 at 9:31 AM Matheus K. Ferst
<matheus.ferst@eldorado.org.br> wrote:
>
> On 14/03/2022 12:20, Khem Raj wrote:
> > On Mon, Mar 14, 2022 at 5:34 AM Matheus K. Ferst
> > <matheus.ferst@eldorado.org.br> wrote:
> >>
> >> On 12/03/2022 23:51, Khem Raj wrote:
> >>> Hi All
> >>>
> >>> I have built an app with musl C library for ppc32 using clang 14.0.0
> >>> and I am using -mcpu=7400 to compile the whole thing including musl.
> >>> This fails to run on qemu usermode reporting illegal instruction, a
> >>> bit of digging revealed that it's caused by opcode in strspn()
> >>> function from musl where clang is emitting vxor instruction and that's
> >>> where qemu ppc bails out.
> >>>
> >>> Here is asm file generated by clang
> >>>
> >>> https://paste.debian.net/1233982/
> >>>
> >>> gcc ( 11.2 )
> >>>
> >>> https://paste.debian.net/1233983/
> >>>
> >>> As you can see the disassembly for gcc does not have vxor or any other
> >>> altivec instructions in it.
> >>> When I add -mno-altivec to clang then the issue is gone or when I use
> >>> -mcpu=power5 the issue is gone too.
> >>>
> >>> This is preprocessed strspn.i
> >>> https://paste.debian.net/1233987/
> >>>
> >>> I wonder if qemu-ppc -mcpu 7400 should have understood vxor instruction 
> >>> or not ?
> >>
> >> Hi Khem,
> >>
> >> A quick test suggests that vxor is working with -cpu 7400:
> >> $ cat > vxor.c << EOF
> >> int main(void)
> >> {
> >>       asm volatile ("vxor 2, 2, 2\n\t" ::: "v2");
> >>       return 0;
> >> }
> >> EOF
> >> $ powerpc64-linux-gnu-gcc -m32 -mcpu=7400 vxor.c -o vxor -static
> >> $ qemu-ppc -cpu 7400 vxor
> >> $ echo $?
> >> 0
> >>
> >> Can you show us the command line you used? What version of QEMU are you
> >> running?
> >
> >
> > I am using musl based toolchain, clang and qemu cmdline is
> > qemu-ppc64le -r 3.10.0 -cpu 7400 ....
> >
>
> Are you building for LE? I've tested strspn with a musl+clang toolchain,
> but it was BE.

Its Big-endian here too

>
> > gdb clearly is at vxor instruction when the system crashes.
>
> If you have debug symbols in QEMU, gen_invalid is a good place to put a
> breakpoint and look around. You can check if gen_vxor is in the
> backtrace,  if ctx->altivec_enabled is set, the opcode being parsed in
> the variable "insn" in ppc_tr_translate_insn, etc.
>
> > I also
> > found another problem
> > with clang when compiling musl itself where plt.got were not correct,
> > maybe there is relation
> > cant say yet.
> >
>
> If you find that it's not only a clang problem, please file a bug on
> GitLab with more information on how to reproduce this issue.

will do thanks for your help.

>
> --
> Matheus K. Ferst
> Instituto de Pesquisas ELDORADO <http://www.eldorado.org.br/>
> Analista de Software
> Aviso Legal - Disclaimer <https://www.eldorado.org.br/disclaimer.html>