Re: [Qemu-devel] [Bug 893208] Re: qemu on ARM hosts can't boot i386 image

[Peter Maydell <peter.maydell@linaro.org>]

On 21 September 2015 at 08:12, Laszlo Ersek <lersek@redhat.com> wrote:
> Where does the division by zero come from then? Well grub fetches and
> stashes the TSC, then programs the PIT to sleep for some time, then
> re-fetches the TSC, and uses the TSC difference as denominator when
> calculating the "TSC rate". (It has a solid idea of the real time
> passed, due to the PIT frequency being a given.)

I was wondering rereading the bug report whether this was down
to our lousy RDTSC implementation...thanks for digging in and
confirming what's going on.

> Now, the cpu_get_real_ticks() implementation is *host* specific. You can
> find it implemented for a bunch of host architectures in
> "include/qemu/timer.h".

> I applied the following extremely sophisticated patch (with the motto
> "it cannot get more wronger"):
>
>> diff --git a/include/qemu/timer.h b/include/qemu/timer.h
>> index 9939246..def22de 100644
>> --- a/include/qemu/timer.h
>> +++ b/include/qemu/timer.h
>> @@ -1003,8 +1003,7 @@ static inline int64_t cpu_get_real_ticks(void)
>>     totally wrong, but hopefully better than nothing.  */
>>  static inline int64_t cpu_get_real_ticks (void)
>>  {
>> -    static int64_t ticks = 0;
>> -    return ticks++;
>> +    return get_clock();
>>  }
>>  #endif
>>
>
> get_clock() is CLOCK_MONOTONIC based, has (theoretical) nanosecond
> resolution, and a nice flat int64_t encoding that should suffice for
> approx. 329 years. This should provide grub with a larger denominator.
>
> This "fix" allowed me to boot the i386 Debian image on the AARCH64 host.
>
> For a real fix... I think on AARCH64 hosts at least, a "real" cycle
> counter should be available, and someone who knows AARCH64 could write a
> function that fetches it.
>
> For 32-bit ARM, I presume the Raspberry Pi 2 and the Odroid C1 are
> advanced enough for a similar cycle counter reading function.

There isn't a user-space readable cycle counter on ARM.
(There is a counter which might be accessible to userspace
depending on kernel config, but the kernel doesn't guarantee
its availability as an ABI thing.)

Probably we should figure out a sane way to emulate guest
cycle counters that isn't dependent on the host CPU architecture.
I think having QEMU's behaviour as seen by the guest vary like
this is a recipe for confusion.

thanks
-- PMM