[GSoC - x86_64] Pre-merge issues (at -O2 optimization level) and WIP review

[Amaan Cheval]

Hi!

I've narrowed the issue down to this bintime function:
https://github.com/RTEMS/rtems/blob/b2de4260c5c71e518742731a8cdebe3411937181/cpukit/score/src/kern_tc.c#L548

The watchdog ticks in _Per_CPU_Information / Clock_driver_ticks are at
"1000", when that function is called (rtems_clock_get_tod ->
_TOD_Get_timeval -> _Timecounter_Microtime -> microtime). The bt and
tvp values there are:

(gdb) p bt
$2 = {sec = 599562004, frac = 18446744073709551536}
(gdb) p *tvp
$3 = {tv_sec = 599562004, tv_usec = 999999}

The full (relevant) debug log for the "wrong" timing despite the
Clock_driver_ticks being correct is here:
https://gist.github.com/AmaanC/c59caf5232b03054d457dcacb5ab1c54

I'm quite unfamiliar with how the low-level internals work and it
looks like it comes from FreeBSD. This is likely a bug from the
timecounter being "too" precise - it dispatches the task at _exactly_
the tc_freq it promised - if it slips by 1 tick, then the values start
looking correct.

This looks more like an off-by-one in the low-level code, in that
case, since my clock driver's timecounter returns exactly the value it
ought to be returning (100 when 1 second has passed, for eg., when the
tc_frequency=100Hz - in that case the bintime's returned "now.tv_sec"
value in clockgettod.c causes the wrong second to be set in
"time_buffer").

https://github.com/AmaanC/rtems-gsoc18/blob/ac/daily-03-post-hello/bsps/x86_64/amd64/clock/clock.c#L51

On Sun, Aug 12, 2018 at 2:48 PM, Amaan Cheval <amaan.cheval at gmail.com> wrote:
> There's another issue I'm having now:
>
> At -O0, ticker.exe works well and passes reliably. At -O2, the TOD
> seems to be rushed a bit:
>
> TA1  - rtems_clock_get_tod - 09:00:00   12/31/1988
> TA2  - rtems_clock_get_tod - 09:00:00   12/31/1988
> TA3  - rtems_clock_get_tod - 09:00:00   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:04   12/31/1988
> TA2  - rtems_clock_get_tod - 09:00:09   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:09   12/31/1988
> TA3  - rtems_clock_get_tod - 09:00:14   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:14   12/31/1988
> TA2  - rtems_clock_get_tod - 09:00:19   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:19   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:24   12/31/1988
> TA3  - rtems_clock_get_tod - 09:00:29   12/31/1988
> TA2  - rtems_clock_get_tod - 09:00:29   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:29   12/31/1988
> TA1  - rtems_clock_get_tod - 09:00:34   12/31/1988
>
> I'm not sure what it could be - I suspected my get_timecount somehow
> not realizing that Clock_driver_ticks was volatile, but that seems to
> be in order. The relevant code is here:
> https://github.com/AmaanC/rtems-gsoc18/blob/ac/daily-03-post-hello/bsps/x86_64/amd64/clock/clock.c
>
> On Sun, Aug 12, 2018 at 3:43 AM, Amaan Cheval <amaan.cheval at gmail.com> wrote:
>> Figured it out; turns out my code to align the stack so I could make
>> calls without raising exceptions was messing up and corrupting the
>> stack-pointer.
>>
>> Running the -O2 code now makes the clock run a bit too quickly - the
>> calibration may have a minor issue. I'll fix that up and send patches
>> tomorrow or Monday hopefully.
>>
>> I'll be traveling Tuesday, so I'd appreciate if we can get them merged
>> upstream Monday itself - I'm okay to have a call and walk someone
>> through the patches and whatnot if need be.
>>
>> Cheers!
>>
>> On Sun, Aug 12, 2018 at 1:25 AM, Amaan Cheval <amaan.cheval at gmail.com> wrote:
>>> Hi!
>>>
>>> In the process of cleaning my work up, I've run into an odd problem
>>> which only shows up when I set the optimization level to -O2. At -O0,
>>> it's perfectly fine.
>>>
>>> The issue is that somehow execution ends up at address 0x0.
>>>
>>> This likely happens due to a _CPU_Context_switch, where the rsp is set
>>> to a corrupted value, leading to a corrupt (i.e. 0) return address at
>>> the end of the context switch.
>>>
>>> What's curious is that this corruption _seems_ to occur in
>>> _ISR_Handler's call to _Thread_Dispatch, by somehow messing the value
>>> of rsp up - I honestly don't know this for sure because gdb says one
>>> thing (i.e. that rsp = 0), but setting up some code (cmpq $0, rsp) to
>>> check this seems to say rsp is non-zero, at least.
>>>
>>> This is an odd heisenbug I'd like to investigate for sure - I just
>>> thought I'd shoot this email out because:
>>>
>>> - If I can't figure it out tomorrow, soon, I'll just drop it so I can
>>> create more logical commits to send as patches upstream (thereby
>>> leaving -O0 upstream, at least temporarily)
>>>
>>> - If anyone's seen an odd stack corruption like this, or has any
>>> advice on debugging it, could you let me know? I suspect something
>>> like interrupting tasks which ought not to be interrupted (perhaps I
>>> forgot to implement some kind of "CPU_ISR_Disable") - is there
>>> anything you can think of of that sort?
>>>
>>> Also, here's a Github PR like last time with all the work (just for
>>> the overall changes, not the specific commits!). I'd appreciate a
>>> quick review if anyone could - sorry about sending this out over the
>>> weekend! I've had a surprising share of Heisenbugs with QEMU in the
>>> past week.
>>>
>>> https://github.com/AmaanC/rtems-gsoc18/pull/3/files