Comment 9 for bug 893208

On 09/21/15 17:50, Dr. David Alan Gilbert wrote:
> * Peter Maydell (<email address hidden>) wrote:
>> On 21 September 2015 at 08:12, Laszlo Ersek <email address hidden> 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.
>
> Time is always hard though; what are the requirements for that
> particular view of time:
>
> 1) It must be monotonic - which get_clock() is iff the host
> supports it (which I guess most do?)
> 2) It's got to be within a few orders of magnitude of sane
> with respect to wall clock, so that if someone measures
> it over a second or a 1/100th of a second or whatever then
> it's still seen to go up.
>
> get_clock() isn't that bad if it's monotonic; if not I'd suggest
> for TCG a multiple of the number of TBs executed (if that's
> already stored somewhere), or something similar.

I think that's quite what -icount does; I had even tested -icount before
posting my email, and it works too. (See -icount in qemu-options.hx.) I
hadn't known about -icount, but I saw the connection in the
cpu_get_ticks() function (mentioned earlier in the call tree):

/* return the host CPU cycle counter and handle stop/restart */
/* Caller must hold the BQL */
int64_t cpu_get_ticks(void)
{
    int64_t ticks;

    if (use_icount) {
        return cpu_get_icount();
    }

...

I didn't recommend it because the documentation in "qemu-options.hx"
confused me, and I thought the emulation should work without obscure
switches.

Thanks
Laszlo

>
> Dave
>
>> thanks
>> -- PMM
> --
> Dr. David Alan Gilbert / <email address hidden> / Manchester, UK
>