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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> 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.

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.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK