Re: qemu-system-s390x hang in tcg (was: Re: [PATCH v8 23/70] mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap())

[Alex Bennée]

Sven Schnelle <svens@xxxxxxxxxxxxx> writes:

> Hi,
>
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
>> On 04.05.22 09:37, Janosch Frank wrote:
>>> I had a short look yesterday and the boot usually hangs in the raid6 
>>> code. Disabling vector instructions didn't make a difference but a few 
>>> interruptions via GDB solve the problem for some reason.
>>> 
>>> CCing David and Thomas for TCG
>>> 
>>
>> I somehow recall that KASAN was always disabled under TCG, I might be
>> wrong (I thought we'd get a message early during boot that the HW
>> doesn't support KASAN).
>>
>> I recall that raid code is a heavy user of vector instructions.
>>
>> How can I reproduce? Compile upstream (or -next?) with kasan support and
>> run it under TCG?
>
> I spent some time looking into this. It's usually hanging in
> s390vx8_gen_syndrome(). My first thought was that it is a problem with
> the VX instructions, but turned out that it hangs even if i remove all
> the code from s390vx8_gen_syndrome().
>
> Tracing the execution of TB's, i see that the generated code is always
> jumping between a few TB's, but never exiting the TB's to check for
> interrupts (i.e. return to cpu_tb_exec(). I only see calls to
> helper_lookup_tb_ptr to lookup the tb pointer for the next TB.
>
> The raid6 code is waiting for some time to expire by reading jiffies,
> but interrupts are never processed and therefore jiffies doesn't change.
> So the raid6 code hangs forever.
>
> As a test, i made a quick change to test:
>
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index c997c2e8e0..35819fd5a7 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -319,7 +319,8 @@ const void *HELPER(lookup_tb_ptr)(CPUArchState *env)
>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
>
>      cflags = curr_cflags(cpu);
> -    if (check_for_breakpoints(cpu, pc, &cflags)) {
> +    if (check_for_breakpoints(cpu, pc, &cflags) ||
> +        unlikely(qatomic_read(&cpu->interrupt_request))) {
>          cpu_loop_exit(cpu);
>      }
>
> And that makes the problem go away. But i'm not familiar with the TCG
> internals, so i can't say whether the generated code is incorrect or
> something else is wrong. I have tcg log files of a failing + working run
> if someone wants to take a look. They are rather large so i would have to
> upload them somewhere.

Whatever is setting cpu->interrupt_request should be calling
cpu_exit(cpu) which sets the exit flag which is checked at the start of
every TB execution (see gen_tb_start).

-- 
Alex Bennée