From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <49A29871.90807@domain.hid> Date: Mon, 23 Feb 2009 13:37:05 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <499EBF45.900@domain.hid> <49A290E8.5030400@domain.hid> In-Reply-To: <49A290E8.5030400@domain.hid> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] [PATCH] x86: Proper root domain state management for ipipe_handle_exception List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: rpm@xenomai.org Cc: xenomai-help , adeos-main Philippe Gerum wrote: > Jan Kiszka wrote: >> This is an attempt to fix the broken root domain state adjustment in >> __ipipe_handle_exception. Patch below fixes the issues recently reported >> by Roman Pisl. Also, it currently makes much more sense to me than what >> we have so far. >> >> In short, this patch propagates the hardware irq state into the root >> domains stall flag shortly before calling into the Linux handler, and >> only then. This avoids spurious root domain stalls the end up over the >> wrong Linux context due to context switches between enter and exit of >> ipipe_handle_exception. Also, this patch drops the bogus >> local_irq_save/restore pair that doesn't account for Linux irq state >> changes inside its fault handler. >> > > Actually, it is not bogus at all, it is even mandatory on x86_64, given that we > don't branch to any sysretq/iretq emulation unlike with x86_32. So if we don't > restore the stall bit for the root domain properly there, we could end up > running with interrupts off in user-space. > > However, the way the interrupt state is currently saved is wrong: we should not > local_irq_disable() over non-root domains. Here is some on-line documentation to > explain why: > > The main difference between x86_32 and 64 is that the former does virtualize the > interrupt state in entry_32.S, unlike the latter. For that reason, x86_64 does > not require (actually, we should not be doing) any fixup. So, to sum up: > > - we use fixup_if() to restore the virtual interrupt state properly when control > is given back to the code that triggered the fault/exception (x86_32). We need > to do that because of task migrations between primary and secondary modes. > > - we must clear the virtual interrupt flag before calling the I-pipe handler / > Linux regular exception handler, because our callee may/must run in the root > domain as well, and expect that interrupt state to reflect the hw one, as set by > the x86 exception gate / fault prologue in entry_*.S. > > - because of the above, we must use local_irq_save()/local_irq_restore_nosync() > in our fault handler to make sure to restore the virtual interrupt flag properly > between this routine, and the exception return statement (i.e. during the Linux > fault epilogue in entry_*.S). OK, if there is a reason to enforce a stalled root domain while calling into the exception hook, this makes some sense. But I don't think it is formally correct to save the root state on entry and blindly restore it _after_ calling the Linux handler. I rather think we should keep the state that Linux leaves behind to remain transparent to it. Maybe no practical issue ATM, but it makes the code at least illogical. Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux