Re: [Xenomai] Mayday mechanism broken on x86-64 - simpler approach feasible?

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2014-12-17 19:28, Philippe Gerum wrote:
> On 12/17/2014 06:46 PM, Jan Kiszka wrote:
>> On 2014-12-17 18:52, Philippe Gerum wrote:
>>> On 12/17/2014 05:57 PM, Jan Kiszka wrote:
>>>> Hi,
>>>>
>>>> while porting the sigdebug test case to forge I noticed occasional
>>>> crashes of the test after it received a watchdog signal. It turned out
>>>> that the problem is in the way mayday works on x86-64:
>>>>
>>>> 1. sp/ip/ax of interrupted thread is saved
>>>> 2. ip is set to mayday page
>>>> 3. mayday page executes the mayday syscall via the syscall instruction
>>>> 4. the syscall restores state on original state on return
>>>>
>>>> That that's the theory. Unfortunately the syscall instructions
>>>> overwrites the cx register with the caller's instruction pointer. We
>>>> could save it but there is no way to restore it on syscall return
>>>> without significant changes to the Linux code in entry_64.S.
>>>>
>>>> x86-32 is fine as it uses the int80 path which does a full state
>>>> recovery - but that is not available for 64-bit (only via ia32 compat,
>>>> but that can be turned off).
>>>>
>>>
>>> The legacy system_call vector is available unconditionally to x86_64.
>>> Xenomai 3 is actually calling int80 unconditionally on mayday traps for
>>> these reasons, i.e. always available and not suffering the long-mode
>>> syscall registers issue.
>>
>> Only when setting CONFIG_IA32_EMULATION. That is typcially the case, but
>> not necessarily.
>>
> 
> I mean: the "safe" Xenomai syscall form (the one that binds a process to
> the real-time core in dual kernel mode) depends on this. Disabling int80
> is not an option in the current Xenomai implementation, mayday feature
> put aside.

Huh? Things worked fine here on X3 when trying out the effect of
disabling IA32 emulation. Or do you mean 2.6?

> 
> So this change would also break the Xenomai ABI (which is ok with x3,
> but not an option for 2.6.4).

Yes, 2.6 requires a compatible solution.

Jan

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