Messages in this thread | | | Date | Tue, 26 Jul 2016 13:56:37 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface |
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On Tue, Jul 26, 2016 at 01:51:27PM -0400, Steven Rostedt wrote: > On Tue, 26 Jul 2016 11:26:42 -0500 > Josh Poimboeuf <jpoimboe@redhat.com> wrote: > > > > Ok, I think that makes sense to me now. As I understand it, the > > "outermost" RIP is the authoritative one, because it was written by the > > original NMI. Any nested NMIs will update the original and/or iret > > RIPs, which will only ever point to NMI entry code, and so they should > > be ignored. > > Just to confirm: > > -- top-of-stack -- > [ hardware written stack ] <- what the NMI hardware mechanism wrote > [ internal variables ] <- you don't need to know what this is > [ where to go next ] <- the stack to use to return on current NMI > [ original copy of hardware stack ] <- the stack of the first NMI > > IIRC, the original version had the "where to go next" stack last, but > to keep pt_regs in line with the stack, it made sense to have the > original NMI stack at the bottom, just above pt_regs, like a real > interrupt would. > > > > > But I think there's a case where this wouldn't work: > > > > task stack > > NMI > > IST > > stack dump > > > > If the IST interrupt hits before the NMI has a chance to update the > > outermost regs, the authoritative RIP would be the original one written > > by HW, right? > > The only IST interrupt that would hit there is MCE and it would > probably be a critical error. Do we really need to worry about such an > unlikely scenario? The system is probably doomed anyway.
According to entry_64.S:
/* * We allow breakpoints in NMIs. If a breakpoint occurs, then * the iretq it performs will take us out of NMI context. * This means that we can have nested NMIs where the next * NMI is using the top of the stack of the previous NMI.
So I think this means that when a debug exception returns to an NMI with iret, further NMIs are no longer masked.
-- Josh
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