Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

[Don Zickus <dzickus@redhat.com>]

On Fri, Sep 02, 2011 at 02:50:53PM -0700, Jeremy Fitzhardinge wrote:
> On 09/02/2011 01:47 PM, Peter Zijlstra wrote:
> > On Fri, 2011-09-02 at 12:29 -0700, Jeremy Fitzhardinge wrote:
> >>> I know that its generally considered bad form, but there's at least one
> >>> spinlock that's only taken from NMI context and thus hasn't got any
> >>> deadlock potential.
> >> Which one? 
> > arch/x86/kernel/traps.c:nmi_reason_lock
> >
> > It serializes NMI access to the NMI reason port across CPUs.
> 
> Ah, OK.  Well, that will never happen in a PV Xen guest.  But PV
> ticketlocks are equally applicable to an HVM Xen domain (and KVM guest),
> so I guess there's at least some chance there could be a virtual
> emulated NMI.  Maybe?  Does qemu do that kind of thing?
> 
> But, erm, does that even make sense?  I'm assuming the NMI reason port
> tells the CPU why it got an NMI.  If multiple CPUs can get NMIs and
> there's only a single reason port, then doesn't that mean that either 1)
> they all got the NMI for the same reason, or 2) having a single port is
> inherently racy?  How does the locking actually work there?

The reason port is for an external/system NMI.  All the IPI-NMI don't need
to access this register to process their handlers, ie perf.  I think in
general the IOAPIC is configured to deliver the external NMI to one cpu,
usually the bsp cpu.  However, there has been a slow movement to free the
bsp cpu from exceptions like this to allow one to eventually hot-swap the
bsp cpu.  The spin locks in that code were an attempt to be more abstract
about who really gets the external NMI.  Of course SGI's box is setup to
deliver an external NMI to all cpus to dump the stack when the system
isn't behaving.

This is a very low usage NMI (in fact almost all cases lead to loud
console messages).

Hope that clears up some of the confusion.

Cheers,
Don