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

On Fri, 2011-09-02 at 14:50 -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?

Afaik qemu/kvm can do the whole NMI thing, yes.

> 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?

I really wouldn't know, the whole NMI thing is a bit of a trainwreck
architecturally. Maybe the x86 maintainers or Linus knows more on this
aspect of it.

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