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

[Avi Kivity <avi@xxxxxxxxxx>]

On 09/06/2011 09:27 PM, Don Zickus wrote:
> On Tue, Sep 06, 2011 at 11:07:26AM -0700, Jeremy Fitzhardinge wrote:
> >  >>  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.
> >
> >  Hm, not really.
> >
> >  What does it mean if two CPUs go down that path?  Should one do some NMI
> >  processing while the other waits around for it to finish, and then do
> >  some NMI processing on its own?
>
> Well the time the second one gets to the external NMI it should have been
> cleared by the first cpu, which would of course lead to the second cpu
> causing a 'Dazed and confused' message.  But on most x86 machines only one
> cpu should be routed the external NMI.  Though there is an SGI box that is
> designed to send an external NMI to all of its cpus.
Is there a way to tell whether an NMI was internally or externally 
generated?
I don't think so, especially as two or more NMIs can be coalesced.  So 
any NMI received on this first cpu has to check the NMI reason port?
> >
> >  But on the other hand, I don't really care if you can say that this path
> >  will never be called in a virtual machine.
>
> Does virtual machines support hot remove of cpus?  Probably not
> considering bare-metal barely supports it.
They do.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.


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