Re: qemu-system-ppc64 option -smp 2 broken with commit 20b6643324a79860dcdfe811ffe4a79942bca21e

[Alex Bennée]

Cédric Le Goater <clg@kaod.org> writes:

> Hello Anushree,
>
> On 6/23/23 13:09, Anushree Mathur wrote:
>> Hi everyone,
>> I was trying to boot rhel9.3 image with upstream qemu-system-ppc64
>> -smp 2 option and observed a segfault (qemu crash).
>> qemu command line used:
>> qemu-system-ppc64 -name Rhel9.3.ppc64le -smp 2 -m 16G -vga none
>> -nographic -machine pseries -cpu POWER10 -accel tcg -device
>> virtio-scsi-pci -drive
>> file=/home/rh93.qcow2,if=none,format=qcow2,id=hd0 -device
>> scsi-hd,drive=hd0 -boot c
>> After doing a git bisect, I found the first bad commit which
>> introduced this issue is below:
>
> Could you please open a gitlab issue on QEMU project ?
>
>  https://gitlab.com/qemu-project/qemu/-/issues

Is it broken generated code that faults or does the goto_tb code break
the execution sequence in some subtle way further down the line?

If you can isolate the guest address the output from:

  -dfilter 0xBADADDR+0x100 -d in_asm,op,out_asm

would be useful for the bug report. Although conceivably the out_asm
output might make sense at translation time and then be broken when it
is patched. Having rr on power would be really useful to debug this sort
of thing. 

>
> Thanks,
>
> C.
>
>> [qemu]# git bisect good
>> 20b6643324a79860dcdfe811ffe4a79942bca21e is the first bad commit
>> commit 20b6643324a79860dcdfe811ffe4a79942bca21e
>> Author: Richard Henderson <richard.henderson@linaro.org>
>> Date:   Mon Dec 5 17:45:02 2022 -0600
>>      tcg/ppc: Reorg goto_tb implementation
>>      The old ppc64 implementation replaces 2 or 4 insns, which
>> leaves a race
>>      condition in which a thread could be stopped at a PC in the middle of
>>      the sequence, and when restarted does not see the complete address
>>      computation and branches to nowhere.
>>      The new implemetation replaces only one insn, swapping between
>>              b       <dest>
>>      and
>>              mtctr   r31
>>      falling through to a general-case indirect branch.
>>      Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>>      Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>>   tcg/ppc/tcg-target.c.inc | 152
>> +++++++++++++----------------------------------
>>   tcg/ppc/tcg-target.h     |   3 +-
>>   2 files changed, 41 insertions(+), 114 deletions(-)
>> [qemu]#
>> Can someone please take a look and suggest a fix to resolve this
>> issue?
>> Thanks in advance.
>> Regards,
>> Anushree-Mathur
>> 


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro