On Fri, Feb 08, 2008 at 11:45:44AM -0500, Vivek Goyal wrote: > On Fri, Feb 08, 2008 at 11:14:22AM -0500, Neil Horman wrote: > > On Thu, Feb 07, 2008 at 01:24:04PM +0100, Ingo Molnar wrote: > > > > > > * Neil Horman <[EMAIL PROTECTED]> wrote: > > > > > > > Ingo noted a few posts down the nmi_exit doesn't actually write to the > > > > APIC EOI register, so yeah, I agree, its bogus (and I apologize, I > > > > should have checked that more carefully). Nevertheless, this patch > > > > consistently allowed a hangning machine to boot through an Nmi lockup. > > > > So I'm forced to wonder whats going on then that this patch helps > > > > with. perhaps its a just a very fragile timing issue, I'll need to > > > > look more closely. > > > > > > try a dummy iret, something like: > > > > > > asm volatile ("pushf; push $1f; iret; 1: \n"); > > > > > > to get the CPU out of its 'nested NMI' state. (totally untested) > > > > > > the idea is to push down an iret frame to the kernel stack that will > > > just jump to the next instruction and gets it out of the NMI nesting. > > > Note: interrupts will/must still be disabled, despite the iret. (the > > > ordering of the pushes might be wrong, we might need more than that for > > > a valid iret, etc. etc.) > > > > > > Ingo > > > > Just tried this experiment and it met with success. Executing a dummy iret > > instruction got us to boot the kdump kernel successfully. > > > > Interesting. So that means there is some operation we can't perform when > we are in NMI handler (Or nested NMIs, I don't know if this is nested NMI > case ). > > Even if we initiated crash dump in NMI handler, next kernel should unlock > that state as soon as we enable interrupts in next kernel (iret will be > called). > > So the only issue here will be if need to put the explicit logic to unlock > the NMI earlier (Either in crashing kernel after clearing IDT or in > purgatory code). Anything earlier then that, will be dangerous though, > handling > another NMI while we are already crashed and doing final preparations to jump > to the new kernel. > > Neil, is it possible to do some serial console debugging to find out > where exactly we are hanging? Beats me, what's that operation which can > not be executed while being in NMI handler and makes system to hang. I am > also curious to know if it is nested NMI case. > I can try, but my last attempts to do so fuond me hung in various places in purgatory or very early in head.S. I'll try again though, to see if I can get some consistency.
Neil > Thanks > Vivek > > _______________________________________________ > kexec mailing list > [EMAIL PROTECTED] > http://lists.infradead.org/mailman/listinfo/kexec -- /*************************************************** *Neil Horman *Software Engineer *Red Hat, Inc. [EMAIL PROTECTED] *gpg keyid: 1024D / 0x92A74FA1 *http://pgp.mit.edu ***************************************************/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/