From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <54DCD68F.5020005@siemens.com> Date: Thu, 12 Feb 2015 17:36:31 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <5491B5E9.4030200@siemens.com> <5491C2CA.7000906@xenomai.org> <5491C179.2000600@siemens.com> <5491CB3F.2050509@xenomai.org> <5491CC27.6080909@siemens.com> <5491D196.3040309@xenomai.org> <54D0E961.4060705@xenomai.org> In-Reply-To: <54D0E961.4060705@xenomai.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] Mayday mechanism broken on x86-64 - simpler approach feasible? List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum , Xenomai On 2015-02-03 16:29, Philippe Gerum wrote: > On 12/17/2014 07:55 PM, Philippe Gerum wrote: >> On 12/17/2014 07:32 PM, Jan Kiszka wrote: >>> 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? >>> >> >> I meant to tell crap: x86_64 implies vsyscall available, so there is no >> "safe" binding prologue there. Only x86_32 has this, so that legacy >> non-NPTL libc which do not support vsyscall work. So back to the initial >> discussion. >> > > As I mentioned earlier, I'd rather fix the MAYDAY implementation for > x86_64 instead of forking the implementation between MMU-enabled and > MMU-less architectures, also affecting powerpc, arm and x86_32 in the > same move. Fortunately, the current implementation allows very specific > tweaks to be applied on a per-architecture basis. This one fixes the > issue for Cobalt on x86_64, and could be easily backported to 2.6.x: > > http://git.xenomai.org/xenomai-3.git/commit/?h=next&id=6db20901963d634b9786467c711c2ba526db48a2 Looks almost good - except for the detail that some bits of the instruction pointer are lost on return from the syscall (int vs. long return type). Patches in the making. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux