Re: [RFC][PATCH 3/3] x86: Add workaround to NMI iret woes

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* 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