Re: [Qemu-devel] [PATCH] cpu-exec(): also reload CPUClass *cc after longjmp return

[Andreas Färber]

Am 05.10.2013 23:45, schrieb Juergen Lock:
> On Sat, Oct 05, 2013 at 08:06:22PM +0200, Stefan Weil wrote:
>> Am 05.10.2013 19:54, schrieb Juergen Lock:
>>> On Fri, Oct 04, 2013 at 09:15:37AM +0200, Jan Kiszka wrote:
>>>> On 2013-10-03 18:05, Peter Maydell wrote:
>>>>> On 3 October 2013 23:09, Juergen Lock <address@hidden> wrote:
>>>>>> Local variable CPUClass *cc needs to be reloaded after return from 
>>>>>> longjmp
>>>>>> too.  (This fixes the mips-softmmu crash observed on FreeBSD when qemu is
>>>>>> built with clang.)
>>>>>>
>>>>>> Signed-off-by: Juergen Lock <address@hidden>
>>>>>> Found-by: Dimitry Andric <address@hidden>
>>>>>>
>>>>>> --- a/cpu-exec.c
>>>>>> +++ b/cpu-exec.c
>>>>>> @@ -681,6 +681,10 @@ int cpu_exec(CPUArchState *env)
>>>>>>               * local variables as longjmp is marked 'noreturn'. */
>>>>>>              cpu = current_cpu;
>>>>>>              env = cpu->env_ptr;
>>>>>> +#if !(defined(CONFIG_USER_ONLY) && \
>>>>>> +      (defined(TARGET_M68K) || defined(TARGET_PPC) || 
>>>>>> defined(TARGET_S390X)))
>>>>>> +            cc = CPU_GET_CLASS(cpu);
>>>>>> +#endif
>>>>> This is a c compiler or libc bug -- the C standard says that this
>>>>> local variable should not be trashed by the longjmp. We were
>>>>> actually discussing removing the current workarounds there...
>>>> But we didn't decide if we should stop supporting the affected compiler
>>>> versions.
>>>>
>>>> Does this issue also exist with the latest clang version available for
>>>> your platform?
>>>>
>>> It happens with up to date clang as it's in FreeBSD 10.0-current
>>> which is due for a release soon.  I think the clang folks are looking
>>> into this issue but I don't know if a fix will make it into the
>>> release...  (For now I've added the workaround to the FreeBSD
>>> qemu-devel port.)
>>>
>>>  Thanx,
>>>     Juergen
>>
>>
>> Could you try whether QEMU crashes when it was configured with
>> TCG interpreter (--enable-tcg-interpreter)? If it does not crash, it
>> might be that TCG does not save / restore enough registers.
>>
> Still crashes the same.

Practical bugfix beats theoretical optimization, so I'm queuing the
patch (w/ message tweaked) until someone comes up with a better one:
https://github.com/afaerber/qemu-cpu/commits/qom-cpu

Thanks,
Andreas

> 
>> Which register is used for the local variable 'cc'?
>>
>  Here is the original debug log with part of the disassembly:
> 
>       http://people.freebsd.org/~nox/tmp/qemu-1.6.0-mips-softmmu-crash.txt
> 
> (I wrote the comment at the top before I knew cc needs to be reloaded...)
> 
>  So apparently cc gets loaded from the stack before the crash: -0x40(%rbp)
> 
>  Thanx,
>       Juergen
> 


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