From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5491D196.3040309@xenomai.org> Date: Wed, 17 Dec 2014 19:55:18 +0100 From: Philippe Gerum 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> In-Reply-To: <5491CC27.6080909@siemens.com> 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: Jan Kiszka , Xenomai 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. -- Philippe.