Messages in this thread | | | Date | Fri, 9 Dec 2011 10:02:49 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC][PATCH 3/3] x86: Add workaround to NMI iret woes |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > On Fri, 2011-12-09 at 08:02 -0500, Mathieu Desnoyers wrote: > > * Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote: > > > after a quick IRC discussion with Peter Zijlstra, one thing seems to be > > missing here to handle the INT3->NMI->INT3 issue: this could be achieved > > by splitting the DEBUG stack in 2 sub-stacks, and letting the int3 > > handler keep track of its nesting within its own stack with an extra > > "int3_nest_count". AFAIU, supporting 2 nested int3 should be enough. > > Here's the problem. When you take an int3, the hardware loads stuff onto > the stack for you. That's the SS, RSP, FLAGS, CS, RIP. If the NMI comes > in while we are processing a breakpoint, and the NMI hits an int3 too, > then the hardware will load the current SS, RSP, FLAGS, CS and RIP onto > the stack at the exact same place as the breakpoint processing that was > interrupted had it's interrupt frame. IOW, it just corrupted the stack. > > To prevent this in the NMI code, I did ugly things like making copies of > the interrupt frame to keep a nested NMI from corrupting the first NMI. > Not only do I not want to do this ugly hack for debug exception, you > *can't* do it. It wont work! > > The reason the NMI works is because while we are copying the stack > frame, NMIs are disabled because we are currently in an NMI. > > But a normal int3, as it tries to do the copy and an NMI triggers, if > you don't update the IDT, any int3 that the NMI hits will corrupt the > previous int3 processing's stack. The hardware does it, there's nothing > a "split stack" will do to fix that.
yep, doing that in the "real" nmi handler (with NMIs disabled at that point) makes tons of sense.
Thanks,
Mathieu
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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