Re: Recoverable MCA interrupts from NMI handlers? IPMI and RCU?

["H. Peter Anvin" <hpa@zytor.com>]

Paul E. McKenney wrote:
> Hello!
> 
> A couple of questions about the x86 architecture...
> 
> 1.	Can recoverable machine-check exceptions occur from within
> 	NMI handlers?  If so, there is a bug in preemptable RCU's
> 	CONFIG_NO_HZ handling that could be fixed by a patch something
> 	like the one shown below (untested, probably does not even
> 	compile).
> 

In theory, recoverable #MC's can occur anywhere.  It's an exception.

> 2.	Does the IPMI subsystem make use of RCU read-side primitives
> 	from within SMI handlers?  If so, we need the SMI handlers to
> 	invoke rcu_irq_enter() upon entry and rcu_irq_exit() upon exit
> 	when they are invoked from dynticks idle state.  Or something
> 	similar, depending on restrictions on code within SMI handlers.

Not sure about that one.  I clearly need to look carefully into this... 
SMI is quite "special" in that it sets up an entirely new environment; 
the concept of executing kernel code at SMI level makes me want to run 
away and hide.

	-hpa